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https://github.com/opentofu/opentofu.git
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034e944070
This is part of a general effort to move all of Terraform's non-library package surface under internal in order to reinforce that these are for internal use within Terraform only. If you were previously importing packages under this prefix into an external codebase, you could pin to an earlier release tag as an interim solution until you've make a plan to achieve the same functionality some other way.
110 lines
2.6 KiB
Go
110 lines
2.6 KiB
Go
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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unmarkedX, xMarks := xs[x].UnmarkDeep()
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unmarkedY, yMarks := ys[y].UnmarkDeep()
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eqV := unmarkedX.Equals(unmarkedY)
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if len(xMarks) != len(yMarks) {
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eqV = cty.False
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}
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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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