105 lines
2.5 KiB
Go
105 lines
2.5 KiB
Go
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package objchange
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import (
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"github.com/zclconf/go-cty/cty"
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)
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// LongestCommonSubsequence finds a sequence of values that are common to both
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// x and y, with the same relative ordering as in both collections. This result
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// is useful as a first step towards computing a diff showing added/removed
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// elements in a sequence.
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//
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// The approached used here is a "naive" one, assuming that both xs and ys will
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// generally be small in most reasonable Terraform configurations. For larger
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// lists the time/space usage may be sub-optimal.
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//
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// A pair of lists may have multiple longest common subsequences. In that
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// case, the one selected by this function is undefined.
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func LongestCommonSubsequence(xs, ys []cty.Value) []cty.Value {
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if len(xs) == 0 || len(ys) == 0 {
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return make([]cty.Value, 0)
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}
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c := make([]int, len(xs)*len(ys))
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eqs := make([]bool, len(xs)*len(ys))
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w := len(xs)
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for y := 0; y < len(ys); y++ {
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for x := 0; x < len(xs); x++ {
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eqV := xs[x].Equals(ys[y])
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eq := false
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if eqV.IsKnown() && eqV.True() {
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eq = true
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eqs[(w*y)+x] = true // equality tests can be expensive, so cache it
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}
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if eq {
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// Sequence gets one longer than for the cell at top left,
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// since we'd append a new item to the sequence here.
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if x == 0 || y == 0 {
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c[(w*y)+x] = 1
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} else {
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c[(w*y)+x] = c[(w*(y-1))+(x-1)] + 1
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}
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} else {
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// We follow the longest of the sequence above and the sequence
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// to the left of us in the matrix.
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l := 0
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u := 0
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if x > 0 {
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l = c[(w*y)+(x-1)]
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}
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if y > 0 {
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u = c[(w*(y-1))+x]
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}
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if l > u {
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c[(w*y)+x] = l
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} else {
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c[(w*y)+x] = u
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}
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}
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}
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}
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// The bottom right cell tells us how long our longest sequence will be
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seq := make([]cty.Value, c[len(c)-1])
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// Now we will walk back from the bottom right cell, finding again all
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// of the equal pairs to construct our sequence.
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x := len(xs) - 1
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y := len(ys) - 1
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i := len(seq) - 1
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for x > -1 && y > -1 {
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if eqs[(w*y)+x] {
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// Add the value to our result list and then walk diagonally
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// up and to the left.
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seq[i] = xs[x]
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x--
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y--
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i--
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} else {
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// Take the path with the greatest sequence length in the matrix.
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l := 0
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u := 0
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if x > 0 {
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l = c[(w*y)+(x-1)]
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}
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if y > 0 {
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u = c[(w*(y-1))+x]
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}
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if l > u {
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x--
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} else {
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y--
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}
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}
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}
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if i > -1 {
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// should never happen if the matrix was constructed properly
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panic("not enough elements in sequence")
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}
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return seq
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}
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