lazygit/pkg/gui/controllers/helpers/diff_line_parser.go

package helpers

import (
	"regexp"
	"strings"

	"github.com/jesseduffield/lazygit/pkg/commands/patch"
	"github.com/jesseduffield/lazygit/pkg/gui/types"
)

// diffFilePrefix marks the start of a file's section in a (possibly multi-file)
// unified diff.
const diffFilePrefix = "diff --git "

// submodulePrefix opens the line a submodule's section of a diff starts with, which
// stands in for the "diff --git" header a file of the repo gets.
const submodulePrefix = "Submodule "

// submoduleSectionPattern matches that line and captures the submodule's path. git
// writes one of two kinds: the commit the submodule is checked out at has moved
// ("Submodule sub a32f27c..2d9f921:", with "..." in place of ".." where the move is
// no fast-forward, " (rewind)" where it goes backwards, and a message in brackets in
// place of the colon where the two commits can't both be read), or its working tree
// is dirty ("Submodule sub contains untracked content").
//
// The path is captured greedily: git writes it unquoted, so a path that itself ends
// in something reading like a range of commits is told apart by taking the last such
// range on the line.
var submoduleSectionPattern = regexp.MustCompile(
	`^Submodule (.+) (?:contains (?:untracked|modified) content|[0-9a-f]+\.{2,3}[0-9a-f]+(?: \(.*\))?:?)$`)

// parsedDiffLine is what the parser recovers about a row of a rendered diff.
// RelPath is the path as the diff header spells it, i.e. relative to the repo
// root; the caller turns it into the absolute path of types.DiffLineInfo.
type parsedDiffLine struct {
	RelPath string
	Type    types.DiffLineType
	NewLine int
	OldLine int
}

// bufferLineParse is the parser's result for one buffer line: the recovered
// identity, and whether the line could be resolved at all (false for a line in
// an unparseable section, or outside any file section).
type bufferLineParse struct {
	parsed parsedDiffLine
	ok     bool
}

// parseDiffLineFromBuffer recovers the identity of a row of a rendered diff by
// parsing the view's decolorized contents.
//
// bufferLines is the full unwrapped view buffer; targetIdx is the buffer line to
// resolve. A commit's diff spans several files, so we isolate the file section
// containing targetIdx and parse just that one (see parseFileSection). Use this
// for a single line, e.g. the one under the cursor; to resolve every line of a
// buffer, use parseAllDiffLinesFromBuffer, which parses each section only once.
//
// ok is false when the buffer isn't a parseable unified diff at targetIdx,
// because the diff renderer restructured it, so that the caller can fall back.
func parseDiffLineFromBuffer(bufferLines []string, targetIdx int) (parsedDiffLine, bool) {
	if targetIdx < 0 || targetIdx >= len(bufferLines) {
		return parsedDiffLine{}, false
	}
	start, end := fileSectionBounds(bufferLines, targetIdx)
	if start == -1 {
		return parsedDiffLine{}, false
	}
	r := parseFileSection(bufferLines[start:end], end == len(bufferLines))[targetIdx-start]
	return r.parsed, r.ok
}

// parseAllDiffLinesFromBuffer resolves every line of a (possibly multi-file)
// diff buffer in one pass, parsing each file section exactly once. It is the
// batch form of parseDiffLineFromBuffer, for callers that scan a whole buffer:
// resolving line by line would re-parse a section once per line of it — O(n²) on
// a large single-file diff — whereas this is O(n). The result is indexed 1:1
// with bufferLines; a line in an unparseable section, or above the first one, is
// left ok=false.
func parseAllDiffLinesFromBuffer(bufferLines []string) []bufferLineParse {
	result := make([]bufferLineParse, len(bufferLines))
	for i := 0; i < len(bufferLines); {
		if !startsFileSection(bufferLines[i]) {
			i++ // in no file section; leave it unresolved
			continue
		}
		end := fileSectionEnd(bufferLines, i)
		copy(result[i:end], parseFileSection(bufferLines[i:end], end == len(bufferLines)))
		i = end
	}
	return result
}

// fileSectionBounds returns the half-open range [start, end) of the file section
// containing targetIdx: the nearest line starting a section at or above it, up to
// where that section ends. start is -1 when targetIdx is above the first file
// section, or below the end of the last one that begins above it.
func fileSectionBounds(bufferLines []string, targetIdx int) (start, end int) {
	for start = targetIdx; start >= 0; start-- {
		if !startsFileSection(bufferLines[start]) {
			continue
		}
		if end = fileSectionEnd(bufferLines, start); targetIdx < end {
			return start, end
		}
		return -1, -1
	}
	return -1, -1
}

// fileSectionEnd returns the line the file section beginning at start ends before.
//
// A file's section runs to the next one, since every line between them is part of
// its diff. A submodule's runs only as far as what git writes for it — the line
// naming it, and the log of the commits it moved over — because the lines after
// that need not belong to any section at all. A diff renderer's output has no
// "diff --git" line to stop at, and the rows it puts between one file and the next
// belong to neither.
func fileSectionEnd(bufferLines []string, start int) int {
	if submodulePath(bufferLines[start]) != "" {
		end := start + 1
		for end < len(bufferLines) && isSubmoduleLogLine(bufferLines[end]) {
			end++
		}
		return end
	}

	for i := start + 1; i < len(bufferLines); i++ {
		if startsFileSection(bufferLines[i]) {
			return i
		}
	}
	return len(bufferLines)
}

// startsFileSection reports whether the line opens a section of a diff: git's header
// for a file of the repo, or the line a submodule's section begins with.
func startsFileSection(line string) bool {
	return strings.HasPrefix(line, diffFilePrefix) || submodulePath(line) != ""
}

// isSubmoduleLogLine reports whether the line is one of the commits git lists under
// a submodule's header, which it writes as two spaces, the direction the commit was
// moved in, and the commit's subject.
func isSubmoduleLogLine(line string) bool {
	return strings.HasPrefix(line, "  > ") || strings.HasPrefix(line, "  < ")
}

// submodulePath returns the submodule whose section the given line opens, and "" for
// every other line.
//
// A submodule gets no "diff --git" header and no hunks: git states which commits it
// moved between and lists them, so that one line is all there is to take the path
// from. The prefix is tested first so that the pattern is run over next to no lines
// of a diff.
func submodulePath(line string) string {
	if !strings.HasPrefix(line, submodulePrefix) {
		return ""
	}
	if match := submoduleSectionPattern.FindStringSubmatch(line); match != nil {
		return match[1]
	}
	return ""
}

// parseFileSection parses one file's diff section (fileLines, starting at the line
// that opens it) a single time and returns the identity of each of its
// lines, indexed 1:1 with fileLines. patch.Parse's line indices line up with the
// section's buffer lines, so the type and the old/new line numbers fall out of
// the patch arithmetic. A submodule's section has no hunks at all, so every row of
// it comes out as a header of the submodule, which is what they are: what git states
// there is which commits it moved between, not lines of a file.
//
// Every line is left ok=false when the section has no
// recoverable path or isn't a well-formed unified diff — the rendering
// restructured it, and acting on a mis-parse would land us on the wrong line, so
// the caller should fall back.
//
// endsTheBuffer says the section runs to the end of what we were given. That is
// where a diff we have only part of breaks off. A long one is read a screenful
// at a time and the rest as the user scrolls, so its last hunk holds fewer lines
// than its header declares until the reading is done. Insisting on the whole
// hunk there would leave every line of the file unresolved while the diff is the
// one on screen, so a section in that position is held to what has arrived.
func parseFileSection(fileLines []string, endsTheBuffer bool) []bufferLineParse {
	result := make([]bufferLineParse, len(fileLines))

	relPath := pathFromDiffHeader(fileLines)
	if relPath == "" {
		return result
	}
	p := patch.Parse(strings.Join(fileLines, "\n"))
	isWellFormed := p.IsWellFormed
	if endsTheBuffer {
		isWellFormed = p.IsWellFormedSoFar
	}
	if !isWellFormed() {
		return result
	}
	patchLines := p.Lines()
	for i := range fileLines {
		if i >= len(patchLines) {
			break
		}
		parsed := parsedDiffLine{
			RelPath: relPath,
			Type:    diffLineTypeForKind(patchLines[i].Kind),
			NewLine: p.LineNumberOfLine(i),
		}
		if parsed.Type == types.DiffLineDeleted {
			parsed.OldLine = p.OldLineNumberOfLine(i)
		}
		result[i] = bufferLineParse{parsed, true}
	}
	return result
}

func diffLineTypeForKind(kind patch.PatchLineKind) types.DiffLineType {
	switch kind {
	case patch.PATCH_HEADER:
		return types.DiffLineFileHeader
	case patch.HUNK_HEADER:
		return types.DiffLineHunkHeader
	case patch.ADDITION:
		return types.DiffLineAdded
	case patch.DELETION:
		return types.DiffLineDeleted
	case patch.CONTEXT:
		return types.DiffLineContext
	default:
		return types.DiffLineOther
	}
}

// pathFromDiffHeader extracts the new-file path of a single diff section. A
// submodule's section states its path in the line it opens with. For a file of the
// repo the path comes from the "+++ b/<path>" line, falling back to "--- a/<path>"
// when the new path is /dev/null (a deleted file), and to the "diff --git" line when
// there are no such lines at all (a pure rename, which has no hunks).
func pathFromDiffHeader(fileLines []string) string {
	if path := submodulePath(fileLines[0]); path != "" {
		return path
	}

	var oldPath, newPath string
	for _, line := range fileLines {
		if strings.HasPrefix(line, "@@") {
			break // past the header
		}
		switch {
		case strings.HasPrefix(line, "+++ "):
			newPath = stripDiffPathPrefix(strings.TrimPrefix(line, "+++ "))
		case strings.HasPrefix(line, "--- "):
			oldPath = stripDiffPathPrefix(strings.TrimPrefix(line, "--- "))
		}
	}

	if newPath != "" && newPath != "/dev/null" {
		return newPath
	}
	if oldPath != "" && oldPath != "/dev/null" {
		return oldPath
	}
	return pathFromDiffGitLine(fileLines[0])
}

// stripDiffPathPrefix removes the a/ or b/ prefix git puts on the paths in a
// diff header. We ask git for these prefixes explicitly (diff.noprefix=false),
// so they are always there.
func stripDiffPathPrefix(path string) string {
	if strings.HasPrefix(path, "a/") || strings.HasPrefix(path, "b/") {
		return path[2:]
	}
	return path
}

// pathFromDiffGitLine extracts the new-file path from a "diff --git a/X b/X"
// line. A path containing " b/" would defeat this, but the +++/--- lines are
// unambiguous and we only get here when they are absent.
func pathFromDiffGitLine(line string) string {
	rest := strings.TrimPrefix(line, diffFilePrefix)
	if idx := strings.LastIndex(rest, " b/"); idx != -1 {
		return rest[idx+len(" b/"):]
	}
	return ""
}