tapir/internal/reconcile/diff.go

package reconcile

// OpKind is the kind of one run in an edit script.
type OpKind int

const (
	// Equal: A[AStart:AEnd] and B[BStart:BEnd] are the same words.
	Equal OpKind = iota
	// Replace: different words on both sides.
	Replace
	// Delete: words only in A.
	Delete
	// Insert: words only in B.
	Insert
)

// Op is one run of the edit script. The ops of a Diff tile both inputs: each op starts
// where the previous one ended, in A and in B.
type Op struct {
	Kind         OpKind
	AStart, AEnd int
	BStart, BEnd int
}

// maxEdits bounds the Myers search. Memory grows with the square of the edit
// distance, so two texts that share almost nothing (one engine produced garbage)
// are reported as one Replace instead of spending gigabytes proving it. A real
// hour disagrees in roughly 400 places, i.e. about 800 edits, far below this.
const maxEdits = 3000

// Diff returns a minimal edit script from a to b (Myers, O(ND)). Adjacent deletions
// and insertions are reported as one Replace.
func Diff(a, b []string) []Op {
	pre := 0
	for pre < len(a) && pre < len(b) && a[pre] == b[pre] {
		pre++
	}
	suf := 0
	for suf < len(a)-pre && suf < len(b)-pre && a[len(a)-1-suf] == b[len(b)-1-suf] {
		suf++
	}
	midA, midB := a[pre:len(a)-suf], b[pre:len(b)-suf]

	steps, ok := myers(midA, midB)
	if !ok {
		steps = steps[:0]
		for range midA {
			steps = append(steps, stepDel)
		}
		for range midB {
			steps = append(steps, stepIns)
		}
	}

	var ops []Op
	ai, bi := 0, 0
	emit := func(k OpKind, da, db int) {
		if da == 0 && db == 0 {
			return
		}
		if k == Equal && len(ops) > 0 && ops[len(ops)-1].Kind == Equal {
			ops[len(ops)-1].AEnd += da
			ops[len(ops)-1].BEnd += db
		} else {
			ops = append(ops, Op{Kind: k, AStart: ai, AEnd: ai + da, BStart: bi, BEnd: bi + db})
		}
		ai += da
		bi += db
	}
	emit(Equal, pre, pre)
	for i := 0; i < len(steps); {
		if steps[i] == stepEq {
			j := i
			for j < len(steps) && steps[j] == stepEq {
				j++
			}
			emit(Equal, j-i, j-i)
			i = j
			continue
		}
		dels, inss := 0, 0
		for i < len(steps) && steps[i] != stepEq {
			if steps[i] == stepDel {
				dels++
			} else {
				inss++
			}
			i++
		}
		switch {
		case dels > 0 && inss > 0:
			emit(Replace, dels, inss)
		case dels > 0:
			emit(Delete, dels, 0)
		default:
			emit(Insert, 0, inss)
		}
	}
	emit(Equal, suf, suf)
	return ops
}

type step int8

const (
	stepEq step = iota
	stepDel
	stepIns
)

// myers returns one step per element of the edit script, in order. ok is false when
// the edit distance exceeds maxEdits.
func myers(a, b []string) ([]step, bool) {
	n, m := len(a), len(b)
	if n == 0 && m == 0 {
		return nil, true
	}
	limit := min(n+m, maxEdits)
	off := limit + 1
	v := make([]int32, 2*limit+3)
	var trace [][]int32 // trace[d] is v as it stood at the start of round d, for k in [-d, d]

	for d := 0; d <= limit; d++ {
		snap := make([]int32, 2*d+1)
		for k := -d; k <= d; k++ {
			snap[k+d] = v[off+k]
		}
		trace = append(trace, snap)
		for k := -d; k <= d; k += 2 {
			var x int
			if k == -d || (k != d && v[off+k-1] < v[off+k+1]) {
				x = int(v[off+k+1])
			} else {
				x = int(v[off+k-1]) + 1
			}
			y := x - k
			for x < n && y < m && a[x] == b[y] {
				x++
				y++
			}
			v[off+k] = int32(x)
			if x >= n && y >= m {
				return backtrack(trace, n, m, d), true
			}
		}
	}
	return nil, false
}

func backtrack(trace [][]int32, n, m, d int) []step {
	var rev []step
	x, y := n, m
	for ; d >= 0; d-- {
		snap := trace[d]
		at := func(k int) int { return int(snap[k+d]) }
		k := x - y
		var prevK int
		if k == -d || (k != d && at(k-1) < at(k+1)) {
			prevK = k + 1
		} else {
			prevK = k - 1
		}
		prevX := 0
		if d > 0 {
			prevX = at(prevK)
		}
		prevY := prevX - prevK
		for x > prevX && y > prevY {
			rev = append(rev, stepEq)
			x--
			y--
		}
		if d > 0 {
			if x == prevX {
				rev = append(rev, stepIns)
			} else {
				rev = append(rev, stepDel)
			}
		}
		x, y = prevX, prevY
	}
	out := make([]step, len(rev))
	for i, s := range rev {
		out[len(rev)-1-i] = s
	}
	return out
}