internal/ztest/diff.go - external/github.com/fsnotify/fsnotify - Git at Google

 // This code is based on https://github.com/pmezard/go-difflib
 //
 // Copyright (c) 2013, Patrick Mezard
 // All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //     Redistributions in binary form must reproduce the above copyright
 // notice, this list of conditions and the following disclaimer in the
 // documentation and/or other materials provided with the distribution.
 //     The names of its contributors may not be used to endorse or promote
 // products derived from this software without specific prior written
 // permission.
 //
 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 // IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 // TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
 // PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 // HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
 // TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 // PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 // LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 // NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 // SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

 // Package ztest is a copy of https://github.com/arp242/zstd/tree/master/ztest –
 // vendored here so we don't add a dependency for just one file used in tests.
 //
 // DiffXML was removed as it depends on zgo.at/zstd/zxml.
 package ztest

 import (
 	"encoding/json"
 	"fmt"
 	"regexp"
 	"strings"
 	"sync"
 	"time"
 )

 type DiffOpt int

 const (
 	// Normalize whitespace: remove all whitespace at the start and end of every
 	// line.
 	DiffNormalizeWhitespace DiffOpt = iota + 1

 	// Treat arguments as JSON: format them before diffing.
 	DiffJSON
 )

 // Diff two strings and format as a unified diff.
 func Diff(have, want string, opt ...DiffOpt) string {
 	have, want = applyOpt(have, want, opt...)

 	d := makeUnifiedDiff(unifiedDiff{
 		A:       splitLines(strings.TrimSpace(have)),
 		B:       splitLines(strings.TrimSpace(want)),
 		Context: 3,
 	})
 	if len(d) == 0 {
 		return ""
 	}
 	return "\n" + d
 }

 // DiffMatch formats a unified diff, but accepts various patterns in the want
 // string:
 //
 //	%(YEAR)      current year in UTC
 //	%(MONTH)     current month in UTC
 //	%(DAY)       current day in UTC
 //	%(UUID)      UUID format (any version).
 //
 //	%(ANY)       any text: .+?
 //	%(ANY 5)     any text of exactly 5 characters: .{5}?
 //	%(ANY 5,)    any text of at least 5 characters: .{5,}?
 //	%(ANY 5,10)  any text between 5 and 10 characters: .{5,10}?
 //	%(ANY 10)    any text at most 10 characters: .{,10}?
 //	%(NUMBER)    any number; also allows length like ANY.
 //
 //	%(..)        any regular expression, but \ is not allowed.
 func DiffMatch(have, want string, opt ...DiffOpt) string {
 	// TODO: %(..) syntax is somewhat unfortunate, as it conflicts with fmt
 	// formatting strings. Would be better to use $(..), #(..), @(..), or
 	// anything else really.
 	have, want = applyOpt(have, want, opt...)

 	now := time.Now().UTC()
 	r := strings.NewReplacer(
 		`%(YEAR)`, fmt.Sprintf("%d", now.Year()),
 		`%(MONTH)`, fmt.Sprintf("%02d", now.Month()),
 		`%(DAY)`, fmt.Sprintf("%02d", now.Day()),
 	)

 	wantRe := regexp.MustCompile(`%\\\(.+?\\\)`).ReplaceAllStringFunc(
 		regexp.QuoteMeta(r.Replace(want)),
 		func(m string) string {
 			switch {
 			case m == `%\(UUID\)`:
 				return `[[:xdigit:]]{8}-[[:xdigit:]]{4}-[[:xdigit:]]{4}-[[:xdigit:]]{4}-[[:xdigit:]]{12}`
 			case m == `%\(ANY\)`:
 				return `.+?`
 			case m == `%\(NUMBER\)`:
 				return `\d+?`
 			case strings.HasPrefix(m, `%\(ANY `):
 				return fmt.Sprintf(`.{%s}?`, m[7:len(m)-2])
 			case strings.HasPrefix(m, `%\(NUMBER `):
 				return fmt.Sprintf(`\d{%s}?`, m[10:len(m)-2])
 			default:
 				// TODO: we need to undo the \ from QuoteMeta() here, but this
 				// means we won't be allowed to use \. Be a bit smarter about
 				// this. TODO: doesn't quite seem to work?
 				return strings.ReplaceAll(m[3:len(m)-2], `\`, ``)
 			}
 		})

 	// Quick check for exact match.
 	if m := regexp.MustCompile(`^` + wantRe + `$`).MatchString(have); m {
 		return ""
 	}

 	diff := unifiedDiff{
 		A:       splitLines(strings.TrimSpace(have)),
 		B:       splitLines(strings.TrimSpace(wantRe)),
 		Context: 3,
 	}
 	m := newMatcher(diff.A, diff.B)
 	m.cmp = func(a, b string) bool { return regexp.MustCompile(b).MatchString(a) }
 	diff.Matcher = m

 	d := makeUnifiedDiff(diff)
 	if len(d) == 0 {
 		return "ztest.DiffMatch: strings didn't match but no diff?" // Should never happen.
 	}
 	return "\n" + d
 }

 var (
 	reNormalizeWhitespace     *regexp.Regexp
 	reNormalizeWhitespaceOnce sync.Once
 )

 func applyOpt(have, want string, opt ...DiffOpt) (string, string) {
 	for _, o := range opt {
 		switch o {
 		case DiffNormalizeWhitespace:
 			reNormalizeWhitespaceOnce.Do(func() {
 				reNormalizeWhitespace = regexp.MustCompile(`(?m)(^\s+|\s+$)`)
 			})
 			have = reNormalizeWhitespace.ReplaceAllString(have, "")
 			want = reNormalizeWhitespace.ReplaceAllString(want, "")
 		case DiffJSON:
 			if have == "" {
 				have = "{}"
 			}
 			if want == "" {
 				want = "{}"
 			}

 			var h any
 			haveJ, err := indentJSON([]byte(have), &h, "", "    ")
 			if err != nil {
 				have = fmt.Sprintf("ztest.Diff: ERROR formatting have: %s\ntext: %s", err, have)
 			} else {
 				have = string(haveJ)
 			}
 			var w any
 			wantJ, err := indentJSON([]byte(want), &w, "", "    ")
 			if err != nil {
 				want = fmt.Sprintf("ztest.Diff: ERROR formatting want: %s\ntext: %s", err, want)
 			} else {
 				want = string(wantJ)
 			}
 		}
 	}
 	return have, want
 }

 type match struct{ A, B, Size int }

 type opCode struct {
 	Tag            byte
 	I1, I2, J1, J2 int
 }

 // sequenceMatcher compares sequence of strings. The basic
 // algorithm predates, and is a little fancier than, an algorithm
 // published in the late 1980's by Ratcliff and Obershelp under the
 // hyperbolic name "gestalt pattern matching".  The basic idea is to find
 // the longest contiguous matching subsequence.
 //
 // Timing:  Basic R-O is cubic time worst case and quadratic time expected
 // case.  sequenceMatcher is quadratic time for the worst case and has
 // expected-case behavior dependent in a complicated way on how many
 // elements the sequences have in common; best case time is linear.
 type sequenceMatcher struct {
 	a, b []string
 	cmp  func(a, b string) bool
 }

 func newMatcher(a, b []string) *sequenceMatcher {
 	return &sequenceMatcher{
 		a:   a,
 		b:   b,
 		cmp: func(a, b string) bool { return a == b },
 	}
 }

 // Find longest matching block in a[alo:ahi] and b[blo:bhi].
 //
 // Return (i,j,k) such that a[i:i+k] is equal to b[j:j+k], where
 //
 //	alo <= i <= i+k <= ahi
 //	blo <= j <= j+k <= bhi
 //
 // and for all (i',j',k') meeting those conditions,
 //
 //	k >= k'
 //	i <= i'
 //	and if i == i', j <= j'
 //
 // In other words, of all maximal matching blocks, return one that
 // starts earliest in a, and of all those maximal matching blocks that
 // start earliest in a, return the one that starts earliest in b.
 //
 // If no blocks match, return (alo, blo, 0).
 func (m *sequenceMatcher) findLongestMatch(alo, ahi, blo, bhi int) match {
 	// Populate line -> index mapping
 	b2j := make(map[string][]int)
 	for i, s := range m.b {
 		b2j[s] = append(b2j[s], i)
 	}

 	// CAUTION:  stripping common prefix or suffix would be incorrect.
 	// E.g.,
 	//    ab
 	//    acab
 	// Longest matching block is "ab", but if common prefix is
 	// stripped, it's "a" (tied with "b").  UNIX(tm) diff does so
 	// strip, so ends up claiming that ab is changed to acab by
 	// inserting "ca" in the middle.  That's minimal but unintuitive:
 	// "it's obvious" that someone inserted "ac" at the front.
 	// Windiff ends up at the same place as diff, but by pairing up
 	// the unique 'b's and then matching the first two 'a's.
 	besti, bestj, bestsize := alo, blo, 0

 	// find longest match. During an iteration of the loop, j2len[j] = length of
 	// longest match ending with a[i-1] and b[j]
 	j2len := map[int]int{}
 	for i := alo; i != ahi; i++ {
 		// look at all instances of a[i] in b.
 		newj2len := map[int]int{}
 		for _, j := range b2j[m.a[i]] {
 			// a[i] matches b[j]
 			if j < blo {
 				continue
 			}
 			if j >= bhi {
 				break
 			}
 			k := j2len[j-1] + 1
 			newj2len[j] = k
 			if k > bestsize {
 				besti, bestj, bestsize = i-k+1, j-k+1, k
 			}
 		}
 		j2len = newj2len
 	}

 	// Extend the best by elements on each end.  In particular, "popular"
 	// elements aren't in b2j, which greatly speeds the inner loop above.
 	for besti > alo && bestj > blo && m.cmp(m.a[besti-1], m.b[bestj-1]) {
 		besti, bestj, bestsize = besti-1, bestj-1, bestsize+1
 	}
 	for besti+bestsize < ahi && bestj+bestsize < bhi && m.cmp(m.a[besti+bestsize], m.b[bestj+bestsize]) {
 		bestsize += 1
 	}

 	return match{A: besti, B: bestj, Size: bestsize}
 }

 // Return list of triples describing matching subsequences.
 //
 // Each triple is of the form (i, j, n), and means that
 // a[i:i+n] == b[j:j+n].  The triples are monotonically increasing in
 // i and in j. It's also guaranteed that if (i, j, n) and (i', j', n') are
 // adjacent triples in the list, and the second is not the last triple in the
 // list, then i+n != i' or j+n != j'. IOW, adjacent triples never describe
 // adjacent equal blocks.
 //
 // The last triple is a dummy, (len(a), len(b), 0), and is the only
 // triple with n==0.
 func (m *sequenceMatcher) matchingBlocks() []match {
 	var matchBlocks func(alo, ahi, blo, bhi int, matched []match) []match
 	matchBlocks = func(alo, ahi, blo, bhi int, matched []match) []match {
 		match := m.findLongestMatch(alo, ahi, blo, bhi)
 		i, j, k := match.A, match.B, match.Size
 		if match.Size > 0 {
 			if alo < i && blo < j {
 				matched = matchBlocks(alo, i, blo, j, matched)
 			}
 			matched = append(matched, match)
 			if i+k < ahi && j+k < bhi {
 				matched = matchBlocks(i+k, ahi, j+k, bhi, matched)
 			}
 		}
 		return matched
 	}
 	matched := matchBlocks(0, len(m.a), 0, len(m.b), nil)

 	// It's possible that we have adjacent equal blocks in the
 	// matching_blocks list now.
 	nonAdjacent := []match{}
 	i1, j1, k1 := 0, 0, 0
 	for _, b := range matched {
 		// Is this block adjacent to i1, j1, k1?
 		i2, j2, k2 := b.A, b.B, b.Size
 		if i1+k1 == i2 && j1+k1 == j2 {
 			// Yes, so collapse them -- this just increases the length of
 			// the first block by the length of the second, and the first
 			// block so lengthened remains the block to compare against.
 			k1 += k2
 		} else {
 			// Not adjacent.  Remember the first block (k1==0 means it's
 			// the dummy we started with), and make the second block the
 			// new block to compare against.
 			if k1 > 0 {
 				nonAdjacent = append(nonAdjacent, match{i1, j1, k1})
 			}
 			i1, j1, k1 = i2, j2, k2
 		}
 	}
 	if k1 > 0 {
 		nonAdjacent = append(nonAdjacent, match{i1, j1, k1})
 	}

 	return append(nonAdjacent, match{len(m.a), len(m.b), 0})
 }

 // Return list of 5-tuples describing how to turn a into b.
 //
 // Each tuple is of the form (tag, i1, i2, j1, j2).  The first tuple
 // has i1 == j1 == 0, and remaining tuples have i1 == the i2 from the
 // tuple preceding it, and likewise for j1 == the previous j2.
 //
 // The tags are characters, with these meanings:
 //
 // 'r' (replace):  a[i1:i2] should be replaced by b[j1:j2]
 //
 // 'd' (delete):   a[i1:i2] should be deleted, j1==j2 in this case.
 //
 // 'i' (insert):   b[j1:j2] should be inserted at a[i1:i1], i1==i2 in this case.
 //
 // 'e' (equal):    a[i1:i2] == b[j1:j2]
 func (m *sequenceMatcher) GetOpCodes() []opCode {
 	matching := m.matchingBlocks()
 	opCodes := make([]opCode, 0, len(matching))

 	var i, j int
 	for _, m := range matching {
 		//  invariant:  we've pumped out correct diffs to change
 		//  a[:i] into b[:j], and the next matching block is
 		//  a[ai:ai+size] == b[bj:bj+size]. So we need to pump
 		//  out a diff to change a[i:ai] into b[j:bj], pump out
 		//  the matching block, and move (i,j) beyond the match
 		ai, bj, size := m.A, m.B, m.Size
 		tag := byte(0)
 		if i < ai && j < bj {
 			tag = 'r'
 		} else if i < ai {
 			tag = 'd'
 		} else if j < bj {
 			tag = 'i'
 		}
 		if tag > 0 {
 			opCodes = append(opCodes, opCode{tag, i, ai, j, bj})
 		}

 		i, j = ai+size, bj+size
 		// the list of matching blocks is terminated by a
 		// sentinel with size 0
 		if size > 0 {
 			opCodes = append(opCodes, opCode{'e', ai, i, bj, j})
 		}
 	}

 	return opCodes
 }

 // Isolate change clusters by eliminating ranges with no changes.
 //
 // Return a generator of groups with up to n lines of context.
 // Each group is in the same format as returned by GetOpCodes().
 func (m *sequenceMatcher) GetGroupedOpCodes(n int) [][]opCode {
 	if n < 0 {
 		n = 3
 	}
 	codes := m.GetOpCodes()
 	if len(codes) == 0 {
 		codes = []opCode{opCode{'e', 0, 1, 0, 1}}
 	}

 	// Fixup leading and trailing groups if they show no changes.
 	if codes[0].Tag == 'e' {
 		c := codes[0]
 		i1, i2, j1, j2 := c.I1, c.I2, c.J1, c.J2
 		codes[0] = opCode{c.Tag, max(i1, i2-n), i2, max(j1, j2-n), j2}
 	}

 	if codes[len(codes)-1].Tag == 'e' {
 		c := codes[len(codes)-1]
 		i1, i2, j1, j2 := c.I1, c.I2, c.J1, c.J2
 		codes[len(codes)-1] = opCode{c.Tag, i1, min(i2, i1+n), j1, min(j2, j1+n)}
 	}

 	nn := n + n
 	groups := [][]opCode{}
 	group := []opCode{}
 	for _, c := range codes {
 		i1, i2, j1, j2 := c.I1, c.I2, c.J1, c.J2
 		// End the current group and start a new one whenever
 		// there is a large range with no changes.
 		if c.Tag == 'e' && i2-i1 > nn {
 			group = append(group, opCode{c.Tag, i1, min(i2, i1+n),
 				j1, min(j2, j1+n)})
 			groups = append(groups, group)
 			group = []opCode{}
 			i1, j1 = max(i1, i2-n), max(j1, j2-n)
 		}
 		group = append(group, opCode{c.Tag, i1, i2, j1, j2})
 	}

 	if len(group) > 0 && !(len(group) == 1 && group[0].Tag == 'e') {
 		groups = append(groups, group)
 	}
 	return groups
 }

 // Convert range to the "ed" format
 func formatRangeUnified(start, stop int) string {
 	// Per the diff spec at http://www.unix.org/single_unix_specification/
 	beginning := start + 1 // lines start numbering with one
 	length := stop - start
 	if length == 1 {
 		return fmt.Sprintf("%d", beginning)
 	}

 	if length == 0 {
 		beginning -= 1 // empty ranges begin at line just before the range
 	}
 	return fmt.Sprintf("%d,%d", beginning, length)
 }

 // Unified diff parameters
 type unifiedDiff struct {
 	A, B    []string
 	Context int
 	Matcher *sequenceMatcher
 }

 // Compare two sequences of lines; generate the delta as a unified diff.
 //
 // Unified diffs are a compact way of showing line changes and a few
 // lines of context.  The number of context lines is set by 'n' which
 // defaults to three.
 //
 // By default, the diff control lines (those with ---, +++, or @@) are
 // created with a trailing newline.  This is helpful so that inputs
 // created from file.readlines() result in diffs that are suitable for
 // file.writelines() since both the inputs and outputs have trailing
 // newlines.
 //
 // For inputs that do not have trailing newlines, set the lineterm
 // argument to "" so that the output will be uniformly newline free.
 //
 // The unidiff format normally has a header for filenames and modification
 // times.  Any or all of these may be specified using strings for
 // 'fromfile', 'tofile', 'fromfiledate', and 'tofiledate'.
 // The modification times are normally expressed in the ISO 8601 format.
 func makeUnifiedDiff(diff unifiedDiff) string {
 	if diff.Matcher == nil {
 		diff.Matcher = newMatcher(diff.A, diff.B)
 	}

 	var (
 		out     strings.Builder
 		started bool
 	)
 	for _, g := range diff.Matcher.GetGroupedOpCodes(diff.Context) {
 		if !started {
 			started = true
 			out.WriteString("--- have\n")
 			out.WriteString("+++ want\n")
 		}

 		first, last := g[0], g[len(g)-1]
 		out.WriteString(fmt.Sprintf("@@ -%s +%s @@\n",
 			formatRangeUnified(first.I1, last.I2),
 			formatRangeUnified(first.J1, last.J2)))

 		for _, c := range g {
 			i1, i2, j1, j2 := c.I1, c.I2, c.J1, c.J2
 			if c.Tag == 'e' {
 				for _, line := range diff.A[i1:i2] {
 					out.WriteString("      " + line)
 				}
 				continue
 			}

 			if c.Tag == 'r' || c.Tag == 'd' {
 				for _, line := range diff.A[i1:i2] {
 					out.WriteString("-have " + line)
 				}
 			}

 			if c.Tag == 'r' || c.Tag == 'i' {
 				for _, line := range diff.B[j1:j2] {
 					out.WriteString("+want " + line)
 				}
 			}
 		}
 	}

 	return out.String()
 }

 func splitLines(s string) []string {
 	lines := strings.SplitAfter(s, "\n")
 	lines[len(lines)-1] += "\n"
 	return lines
 }

 func indentJSON(data []byte, v any, prefix, indent string) ([]byte, error) {
 	err := json.Unmarshal(data, v)
 	if err != nil {
 		return nil, err
 	}
 	return json.MarshalIndent(v, prefix, indent)
 }
	// This code is based on https://github.com/pmezard/go-difflib
	//
	// Copyright (c) 2013, Patrick Mezard
	// All rights reserved.
	//
	// Redistribution and use in source and binary forms, with or without
	// modification, are permitted provided that the following conditions are
	// met:
	//
	// Redistributions of source code must retain the above copyright
	// notice, this list of conditions and the following disclaimer.
	// Redistributions in binary form must reproduce the above copyright
	// notice, this list of conditions and the following disclaimer in the
	// documentation and/or other materials provided with the distribution.
	// The names of its contributors may not be used to endorse or promote
	// products derived from this software without specific prior written
	// permission.
	//
	// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
	// IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
	// TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
	// PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
	// HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
	// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
	// TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
	// PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
	// LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
	// NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
	// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

	// Package ztest is a copy of https://github.com/arp242/zstd/tree/master/ztest –
	// vendored here so we don't add a dependency for just one file used in tests.
	//
	// DiffXML was removed as it depends on zgo.at/zstd/zxml.
	package ztest

	import (
	"encoding/json"
	"fmt"
	"regexp"
	"strings"
	"sync"
	"time"
	)

	type DiffOpt int

	const (
	// Normalize whitespace: remove all whitespace at the start and end of every
	// line.
	DiffNormalizeWhitespace DiffOpt = iota + 1

	// Treat arguments as JSON: format them before diffing.
	DiffJSON
	)

	// Diff two strings and format as a unified diff.
	func Diff(have, want string, opt ...DiffOpt) string {
	have, want = applyOpt(have, want, opt...)

	d := makeUnifiedDiff(unifiedDiff{
	A: splitLines(strings.TrimSpace(have)),
	B: splitLines(strings.TrimSpace(want)),
	Context: 3,
	})
	if len(d) == 0 {
	return ""
	}
	return "\n" + d
	}

	// DiffMatch formats a unified diff, but accepts various patterns in the want
	// string:
	//
	// %(YEAR) current year in UTC
	// %(MONTH) current month in UTC
	// %(DAY) current day in UTC
	// %(UUID) UUID format (any version).
	//
	// %(ANY) any text: .+?
	// %(ANY 5) any text of exactly 5 characters: .{5}?
	// %(ANY 5,) any text of at least 5 characters: .{5,}?
	// %(ANY 5,10) any text between 5 and 10 characters: .{5,10}?
	// %(ANY 10) any text at most 10 characters: .{,10}?
	// %(NUMBER) any number; also allows length like ANY.
	//
	// %(..) any regular expression, but \ is not allowed.
	func DiffMatch(have, want string, opt ...DiffOpt) string {
	// TODO: %(..) syntax is somewhat unfortunate, as it conflicts with fmt
	// formatting strings. Would be better to use $(..), #(..), @(..), or
	// anything else really.
	have, want = applyOpt(have, want, opt...)

	now := time.Now().UTC()
	r := strings.NewReplacer(
	`%(YEAR)`, fmt.Sprintf("%d", now.Year()),
	`%(MONTH)`, fmt.Sprintf("%02d", now.Month()),
	`%(DAY)`, fmt.Sprintf("%02d", now.Day()),
	)

	wantRe := regexp.MustCompile(`%\\\(.+?\\\)`).ReplaceAllStringFunc(
	regexp.QuoteMeta(r.Replace(want)),
	func(m string) string {
	switch {
	case m == `%\(UUID\)`:
	return `[[:xdigit:]]{8}-[[:xdigit:]]{4}-[[:xdigit:]]{4}-[[:xdigit:]]{4}-[[:xdigit:]]{12}`
	case m == `%\(ANY\)`:
	return `.+?`
	case m == `%\(NUMBER\)`:
	return `\d+?`
	case strings.HasPrefix(m, `%\(ANY `):
	return fmt.Sprintf(`.{%s}?`, m[7:len(m)-2])
	case strings.HasPrefix(m, `%\(NUMBER `):
	return fmt.Sprintf(`\d{%s}?`, m[10:len(m)-2])
	default:
	// TODO: we need to undo the \ from QuoteMeta() here, but this
	// means we won't be allowed to use \. Be a bit smarter about
	// this. TODO: doesn't quite seem to work?
	return strings.ReplaceAll(m[3:len(m)-2], `\`, ``)
	}
	})

	// Quick check for exact match.
	if m := regexp.MustCompile(`^` + wantRe + `$`).MatchString(have); m {
	return ""
	}

	diff := unifiedDiff{
	A: splitLines(strings.TrimSpace(have)),
	B: splitLines(strings.TrimSpace(wantRe)),
	Context: 3,
	}
	m := newMatcher(diff.A, diff.B)
	m.cmp = func(a, b string) bool { return regexp.MustCompile(b).MatchString(a) }
	diff.Matcher = m

	d := makeUnifiedDiff(diff)
	if len(d) == 0 {
	return "ztest.DiffMatch: strings didn't match but no diff?" // Should never happen.
	}
	return "\n" + d
	}

	var (
	reNormalizeWhitespace *regexp.Regexp
	reNormalizeWhitespaceOnce sync.Once
	)

	func applyOpt(have, want string, opt ...DiffOpt) (string, string) {
	for _, o := range opt {
	switch o {
	case DiffNormalizeWhitespace:
	reNormalizeWhitespaceOnce.Do(func() {
	reNormalizeWhitespace = regexp.MustCompile(`(?m)(^\s+\|\s+$)`)
	})
	have = reNormalizeWhitespace.ReplaceAllString(have, "")
	want = reNormalizeWhitespace.ReplaceAllString(want, "")
	case DiffJSON:
	if have == "" {
	have = "{}"
	}
	if want == "" {
	want = "{}"
	}

	var h any
	haveJ, err := indentJSON([]byte(have), &h, "", " ")
	if err != nil {
	have = fmt.Sprintf("ztest.Diff: ERROR formatting have: %s\ntext: %s", err, have)
	} else {
	have = string(haveJ)
	}
	var w any
	wantJ, err := indentJSON([]byte(want), &w, "", " ")
	if err != nil {
	want = fmt.Sprintf("ztest.Diff: ERROR formatting want: %s\ntext: %s", err, want)
	} else {
	want = string(wantJ)
	}
	}
	}
	return have, want
	}

	type match struct{ A, B, Size int }

	type opCode struct {
	Tag byte
	I1, I2, J1, J2 int
	}

	// sequenceMatcher compares sequence of strings. The basic
	// algorithm predates, and is a little fancier than, an algorithm
	// published in the late 1980's by Ratcliff and Obershelp under the
	// hyperbolic name "gestalt pattern matching". The basic idea is to find
	// the longest contiguous matching subsequence.
	//
	// Timing: Basic R-O is cubic time worst case and quadratic time expected
	// case. sequenceMatcher is quadratic time for the worst case and has
	// expected-case behavior dependent in a complicated way on how many
	// elements the sequences have in common; best case time is linear.
	type sequenceMatcher struct {
	a, b []string
	cmp func(a, b string) bool
	}

	func newMatcher(a, b []string) *sequenceMatcher {
	return &sequenceMatcher{
	a: a,
	b: b,
	cmp: func(a, b string) bool { return a == b },
	}
	}

	// Find longest matching block in a[alo:ahi] and b[blo:bhi].
	//
	// Return (i,j,k) such that a[i:i+k] is equal to b[j:j+k], where
	//
	// alo <= i <= i+k <= ahi
	// blo <= j <= j+k <= bhi
	//
	// and for all (i',j',k') meeting those conditions,
	//
	// k >= k'
	// i <= i'
	// and if i == i', j <= j'
	//
	// In other words, of all maximal matching blocks, return one that
	// starts earliest in a, and of all those maximal matching blocks that
	// start earliest in a, return the one that starts earliest in b.
	//
	// If no blocks match, return (alo, blo, 0).
	func (m *sequenceMatcher) findLongestMatch(alo, ahi, blo, bhi int) match {
	// Populate line -> index mapping
	b2j := make(map[string][]int)
	for i, s := range m.b {
	b2j[s] = append(b2j[s], i)
	}

	// CAUTION: stripping common prefix or suffix would be incorrect.
	// E.g.,
	// ab
	// acab
	// Longest matching block is "ab", but if common prefix is
	// stripped, it's "a" (tied with "b"). UNIX(tm) diff does so
	// strip, so ends up claiming that ab is changed to acab by
	// inserting "ca" in the middle. That's minimal but unintuitive:
	// "it's obvious" that someone inserted "ac" at the front.
	// Windiff ends up at the same place as diff, but by pairing up
	// the unique 'b's and then matching the first two 'a's.
	besti, bestj, bestsize := alo, blo, 0

	// find longest match. During an iteration of the loop, j2len[j] = length of
	// longest match ending with a[i-1] and b[j]
	j2len := map[int]int{}
	for i := alo; i != ahi; i++ {
	// look at all instances of a[i] in b.
	newj2len := map[int]int{}
	for _, j := range b2j[m.a[i]] {
	// a[i] matches b[j]
	if j < blo {
	continue
	}
	if j >= bhi {
	break
	}
	k := j2len[j-1] + 1
	newj2len[j] = k
	if k > bestsize {
	besti, bestj, bestsize = i-k+1, j-k+1, k
	}
	}
	j2len = newj2len
	}

	// Extend the best by elements on each end. In particular, "popular"
	// elements aren't in b2j, which greatly speeds the inner loop above.
	for besti > alo && bestj > blo && m.cmp(m.a[besti-1], m.b[bestj-1]) {
	besti, bestj, bestsize = besti-1, bestj-1, bestsize+1
	}
	for besti+bestsize < ahi && bestj+bestsize < bhi && m.cmp(m.a[besti+bestsize], m.b[bestj+bestsize]) {
	bestsize += 1
	}

	return match{A: besti, B: bestj, Size: bestsize}
	}

	// Return list of triples describing matching subsequences.
	//
	// Each triple is of the form (i, j, n), and means that
	// a[i:i+n] == b[j:j+n]. The triples are monotonically increasing in
	// i and in j. It's also guaranteed that if (i, j, n) and (i', j', n') are
	// adjacent triples in the list, and the second is not the last triple in the
	// list, then i+n != i' or j+n != j'. IOW, adjacent triples never describe
	// adjacent equal blocks.
	//
	// The last triple is a dummy, (len(a), len(b), 0), and is the only
	// triple with n==0.
	func (m *sequenceMatcher) matchingBlocks() []match {
	var matchBlocks func(alo, ahi, blo, bhi int, matched []match) []match
	matchBlocks = func(alo, ahi, blo, bhi int, matched []match) []match {
	match := m.findLongestMatch(alo, ahi, blo, bhi)
	i, j, k := match.A, match.B, match.Size
	if match.Size > 0 {
	if alo < i && blo < j {
	matched = matchBlocks(alo, i, blo, j, matched)
	}
	matched = append(matched, match)
	if i+k < ahi && j+k < bhi {
	matched = matchBlocks(i+k, ahi, j+k, bhi, matched)
	}
	}
	return matched
	}
	matched := matchBlocks(0, len(m.a), 0, len(m.b), nil)

	// It's possible that we have adjacent equal blocks in the
	// matching_blocks list now.
	nonAdjacent := []match{}
	i1, j1, k1 := 0, 0, 0
	for _, b := range matched {
	// Is this block adjacent to i1, j1, k1?
	i2, j2, k2 := b.A, b.B, b.Size
	if i1+k1 == i2 && j1+k1 == j2 {
	// Yes, so collapse them -- this just increases the length of
	// the first block by the length of the second, and the first
	// block so lengthened remains the block to compare against.
	k1 += k2
	} else {
	// Not adjacent. Remember the first block (k1==0 means it's
	// the dummy we started with), and make the second block the
	// new block to compare against.
	if k1 > 0 {
	nonAdjacent = append(nonAdjacent, match{i1, j1, k1})
	}
	i1, j1, k1 = i2, j2, k2
	}
	}
	if k1 > 0 {
	nonAdjacent = append(nonAdjacent, match{i1, j1, k1})
	}

	return append(nonAdjacent, match{len(m.a), len(m.b), 0})
	}

	// Return list of 5-tuples describing how to turn a into b.
	//
	// Each tuple is of the form (tag, i1, i2, j1, j2). The first tuple
	// has i1 == j1 == 0, and remaining tuples have i1 == the i2 from the
	// tuple preceding it, and likewise for j1 == the previous j2.
	//
	// The tags are characters, with these meanings:
	//
	// 'r' (replace): a[i1:i2] should be replaced by b[j1:j2]
	//
	// 'd' (delete): a[i1:i2] should be deleted, j1==j2 in this case.
	//
	// 'i' (insert): b[j1:j2] should be inserted at a[i1:i1], i1==i2 in this case.
	//
	// 'e' (equal): a[i1:i2] == b[j1:j2]
	func (m *sequenceMatcher) GetOpCodes() []opCode {
	matching := m.matchingBlocks()
	opCodes := make([]opCode, 0, len(matching))

	var i, j int
	for _, m := range matching {
	// invariant: we've pumped out correct diffs to change
	// a[:i] into b[:j], and the next matching block is
	// a[ai:ai+size] == b[bj:bj+size]. So we need to pump
	// out a diff to change a[i:ai] into b[j:bj], pump out
	// the matching block, and move (i,j) beyond the match
	ai, bj, size := m.A, m.B, m.Size
	tag := byte(0)
	if i < ai && j < bj {
	tag = 'r'
	} else if i < ai {
	tag = 'd'
	} else if j < bj {
	tag = 'i'
	}
	if tag > 0 {
	opCodes = append(opCodes, opCode{tag, i, ai, j, bj})
	}

	i, j = ai+size, bj+size
	// the list of matching blocks is terminated by a
	// sentinel with size 0
	if size > 0 {
	opCodes = append(opCodes, opCode{'e', ai, i, bj, j})
	}
	}

	return opCodes
	}

	// Isolate change clusters by eliminating ranges with no changes.
	//
	// Return a generator of groups with up to n lines of context.
	// Each group is in the same format as returned by GetOpCodes().
	func (m *sequenceMatcher) GetGroupedOpCodes(n int) [][]opCode {
	if n < 0 {
	n = 3
	}
	codes := m.GetOpCodes()
	if len(codes) == 0 {
	codes = []opCode{opCode{'e', 0, 1, 0, 1}}
	}

	// Fixup leading and trailing groups if they show no changes.
	if codes[0].Tag == 'e' {
	c := codes[0]
	i1, i2, j1, j2 := c.I1, c.I2, c.J1, c.J2
	codes[0] = opCode{c.Tag, max(i1, i2-n), i2, max(j1, j2-n), j2}
	}

	if codes[len(codes)-1].Tag == 'e' {
	c := codes[len(codes)-1]
	i1, i2, j1, j2 := c.I1, c.I2, c.J1, c.J2
	codes[len(codes)-1] = opCode{c.Tag, i1, min(i2, i1+n), j1, min(j2, j1+n)}
	}

	nn := n + n
	groups := [][]opCode{}
	group := []opCode{}
	for _, c := range codes {
	i1, i2, j1, j2 := c.I1, c.I2, c.J1, c.J2
	// End the current group and start a new one whenever
	// there is a large range with no changes.
	if c.Tag == 'e' && i2-i1 > nn {
	group = append(group, opCode{c.Tag, i1, min(i2, i1+n),
	j1, min(j2, j1+n)})
	groups = append(groups, group)
	group = []opCode{}
	i1, j1 = max(i1, i2-n), max(j1, j2-n)
	}
	group = append(group, opCode{c.Tag, i1, i2, j1, j2})
	}

	if len(group) > 0 && !(len(group) == 1 && group[0].Tag == 'e') {
	groups = append(groups, group)
	}
	return groups
	}

	// Convert range to the "ed" format
	func formatRangeUnified(start, stop int) string {
	// Per the diff spec at http://www.unix.org/single_unix_specification/
	beginning := start + 1 // lines start numbering with one
	length := stop - start
	if length == 1 {
	return fmt.Sprintf("%d", beginning)
	}

	if length == 0 {
	beginning -= 1 // empty ranges begin at line just before the range
	}
	return fmt.Sprintf("%d,%d", beginning, length)
	}

	// Unified diff parameters
	type unifiedDiff struct {
	A, B []string
	Context int
	Matcher *sequenceMatcher
	}

	// Compare two sequences of lines; generate the delta as a unified diff.
	//
	// Unified diffs are a compact way of showing line changes and a few
	// lines of context. The number of context lines is set by 'n' which
	// defaults to three.
	//
	// By default, the diff control lines (those with ---, +++, or @@) are
	// created with a trailing newline. This is helpful so that inputs
	// created from file.readlines() result in diffs that are suitable for
	// file.writelines() since both the inputs and outputs have trailing
	// newlines.
	//
	// For inputs that do not have trailing newlines, set the lineterm
	// argument to "" so that the output will be uniformly newline free.
	//
	// The unidiff format normally has a header for filenames and modification
	// times. Any or all of these may be specified using strings for
	// 'fromfile', 'tofile', 'fromfiledate', and 'tofiledate'.
	// The modification times are normally expressed in the ISO 8601 format.
	func makeUnifiedDiff(diff unifiedDiff) string {
	if diff.Matcher == nil {
	diff.Matcher = newMatcher(diff.A, diff.B)
	}

	var (
	out strings.Builder
	started bool
	)
	for _, g := range diff.Matcher.GetGroupedOpCodes(diff.Context) {
	if !started {
	started = true
	out.WriteString("--- have\n")
	out.WriteString("+++ want\n")
	}

	first, last := g[0], g[len(g)-1]
	out.WriteString(fmt.Sprintf("@@ -%s +%s @@\n",
	formatRangeUnified(first.I1, last.I2),
	formatRangeUnified(first.J1, last.J2)))

	for _, c := range g {
	i1, i2, j1, j2 := c.I1, c.I2, c.J1, c.J2
	if c.Tag == 'e' {
	for _, line := range diff.A[i1:i2] {
	out.WriteString(" " + line)
	}
	continue
	}

	if c.Tag == 'r' \|\| c.Tag == 'd' {
	for _, line := range diff.A[i1:i2] {
	out.WriteString("-have " + line)
	}
	}

	if c.Tag == 'r' \|\| c.Tag == 'i' {
	for _, line := range diff.B[j1:j2] {
	out.WriteString("+want " + line)
	}
	}
	}
	}

	return out.String()
	}

	func splitLines(s string) []string {
	lines := strings.SplitAfter(s, "\n")
	lines[len(lines)-1] += "\n"
	return lines
	}

	func indentJSON(data []byte, v any, prefix, indent string) ([]byte, error) {
	err := json.Unmarshal(data, v)
	if err != nil {
	return nil, err
	}
	return json.MarshalIndent(v, prefix, indent)
	}