diff.go

     1  // Copyright 2019 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package diff computes differences between text files or strings.
     6  package diff
     7  
     8  import (
     9  	"fmt"
    10  	"slices"
    11  	"sort"
    12  	"strings"
    13  )
    14  
    15  // An Edit describes the replacement of a portion of a text file.
    16  type Edit struct {
    17  	Start, End int    // byte offsets of the region to replace
    18  	New        string // the replacement
    19  }
    20  
    21  func (e Edit) String() string {
    22  	return fmt.Sprintf("{Start:%d,End:%d,New:%q}", e.Start, e.End, e.New)
    23  }
    24  
    25  // Apply applies a sequence of edits to the src buffer and returns the
    26  // result. Edits are applied in order of start offset; edits with the
    27  // same start offset are applied in they order they were provided.
    28  //
    29  // Apply returns an error if any edit is out of bounds,
    30  // or if any pair of edits is overlapping.
    31  func Apply(src string, edits []Edit) (string, error) {
    32  	edits, size, err := validate(src, edits)
    33  	if err != nil {
    34  		return "", err
    35  	}
    36  
    37  	// Apply edits.
    38  	out := make([]byte, 0, size)
    39  	lastEnd := 0
    40  	for _, edit := range edits {
    41  		if lastEnd < edit.Start {
    42  			out = append(out, src[lastEnd:edit.Start]...)
    43  		}
    44  		out = append(out, edit.New...)
    45  		lastEnd = edit.End
    46  	}
    47  	out = append(out, src[lastEnd:]...)
    48  
    49  	if len(out) != size {
    50  		panic("wrong size")
    51  	}
    52  
    53  	return string(out), nil
    54  }
    55  
    56  // ApplyBytes is like Apply, but it accepts a byte slice.
    57  // The result is always a new array.
    58  func ApplyBytes(src []byte, edits []Edit) ([]byte, error) {
    59  	res, err := Apply(string(src), edits)
    60  	return []byte(res), err
    61  }
    62  
    63  // validate checks that edits are consistent with src,
    64  // and returns the size of the patched output.
    65  // It may return a different slice.
    66  func validate(src string, edits []Edit) ([]Edit, int, error) {
    67  	if !sort.IsSorted(editsSort(edits)) {
    68  		edits = slices.Clone(edits)
    69  		SortEdits(edits)
    70  	}
    71  
    72  	// Check validity of edits and compute final size.
    73  	size := len(src)
    74  	lastEnd := 0
    75  	for _, edit := range edits {
    76  		if !(0 <= edit.Start && edit.Start <= edit.End && edit.End <= len(src)) {
    77  			return nil, 0, fmt.Errorf("diff has out-of-bounds edits")
    78  		}
    79  		if edit.Start < lastEnd {
    80  			return nil, 0, fmt.Errorf("diff has overlapping edits")
    81  		}
    82  		size += len(edit.New) + edit.Start - edit.End
    83  		lastEnd = edit.End
    84  	}
    85  
    86  	return edits, size, nil
    87  }
    88  
    89  // SortEdits orders a slice of Edits by (start, end) offset.
    90  // This ordering puts insertions (end = start) before deletions
    91  // (end > start) at the same point, but uses a stable sort to preserve
    92  // the order of multiple insertions at the same point.
    93  // (Apply detects multiple deletions at the same point as an error.)
    94  func SortEdits(edits []Edit) {
    95  	sort.Stable(editsSort(edits))
    96  }
    97  
    98  type editsSort []Edit
    99  
   100  func (a editsSort) Len() int { return len(a) }
   101  func (a editsSort) Less(i, j int) bool {
   102  	if cmp := a[i].Start - a[j].Start; cmp != 0 {
   103  		return cmp < 0
   104  	}
   105  	return a[i].End < a[j].End
   106  }
   107  func (a editsSort) Swap(i, j int) { a[i], a[j] = a[j], a[i] }
   108  
   109  // lineEdits expands and merges a sequence of edits so that each
   110  // resulting edit replaces one or more complete lines.
   111  // See ApplyEdits for preconditions.
   112  func lineEdits(src string, edits []Edit) ([]Edit, error) {
   113  	edits, _, err := validate(src, edits)
   114  	if err != nil {
   115  		return nil, err
   116  	}
   117  
   118  	// Do all deletions begin and end at the start of a line,
   119  	// and all insertions end with a newline?
   120  	// (This is merely a fast path.)
   121  	for _, edit := range edits {
   122  		if edit.Start >= len(src) || // insertion at EOF
   123  			edit.Start > 0 && src[edit.Start-1] != '\n' || // not at line start
   124  			edit.End > 0 && src[edit.End-1] != '\n' || // not at line start
   125  			edit.New != "" && edit.New[len(edit.New)-1] != '\n' { // partial insert
   126  			goto expand // slow path
   127  		}
   128  	}
   129  	return edits, nil // aligned
   130  
   131  expand:
   132  	if len(edits) == 0 {
   133  		return edits, nil // no edits (unreachable due to fast path)
   134  	}
   135  	expanded := make([]Edit, 0, len(edits)) // a guess
   136  	prev := edits[0]
   137  	// TODO(adonovan): opt: start from the first misaligned edit.
   138  	// TODO(adonovan): opt: avoid quadratic cost of string += string.
   139  	for _, edit := range edits[1:] {
   140  		between := src[prev.End:edit.Start]
   141  		if !strings.Contains(between, "\n") {
   142  			// overlapping lines: combine with previous edit.
   143  			prev.New += between + edit.New
   144  			prev.End = edit.End
   145  		} else {
   146  			// non-overlapping lines: flush previous edit.
   147  			expanded = append(expanded, expandEdit(prev, src))
   148  			prev = edit
   149  		}
   150  	}
   151  	return append(expanded, expandEdit(prev, src)), nil // flush final edit
   152  }
   153  
   154  // expandEdit returns edit expanded to complete whole lines.
   155  func expandEdit(edit Edit, src string) Edit {
   156  	// Expand start left to start of line.
   157  	// (delta is the zero-based column number of start.)
   158  	start := edit.Start
   159  	if delta := start - 1 - strings.LastIndex(src[:start], "\n"); delta > 0 {
   160  		edit.Start -= delta
   161  		edit.New = src[start-delta:start] + edit.New
   162  	}
   163  
   164  	// Expand end right to end of line.
   165  	end := edit.End
   166  	if end > 0 && src[end-1] != '\n' ||
   167  		edit.New != "" && edit.New[len(edit.New)-1] != '\n' {
   168  		if nl := strings.IndexByte(src[end:], '\n'); nl < 0 {
   169  			edit.End = len(src) // extend to EOF
   170  		} else {
   171  			edit.End = end + nl + 1 // extend beyond \n
   172  		}
   173  	}
   174  	edit.New += src[end:edit.End]
   175  
   176  	return edit
   177  }
   178
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