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