// Produce all valid "left" indices for a particular search query.
// What determines whether an index is valid is whether it can form the start
// of a complete word. This basically means "any cell with nothing on the
// left".
//
// A cell that has *something* on the left of it cannot form the beginning of a
// word because it must have a prefix (the stuff on the left) to be a word.
//
// Given a query like ..AD.., this will produce the indices 0, 1, 2, 5
// (because indices 3 and 4 are 'D' and the '.' after it, which can't be the
// beginning of a word, but index 2 can, since a word can start with 'AD' in
// this scenario).
func GetSubSuffixes(info index.AllowedInfo) <-chan int {
out := make(chan int)
emit := func(left int) {
if info.AnchoredSubSequence(left, len(info.Draws)) {
out <- left
}
}
go func() {
defer close(out)
for left := 0; left < len(info.Constraints); left++ {
emit(left)
for left < len(info.Constraints) && !info.Draws[left] {
left++
}
}
}()
return out
}
// Get all allowed subconstraints from an initial constraint.
//
// This constitutes peeling off from the left and right whatever can be peeled
// off to form a new sub constraint.
func GetSubConstraints(info index.AllowedInfo) <-chan Endpoints {
out := make(chan Endpoints)
emit := func(left, right int) {
if info.AnchoredSubSequence(left, right) {
out <- Endpoints{left, right}
}
}
go func() {
defer close(out)
for left := range GetSubSuffixes(info) {
emit(left, len(info.Constraints))
for right := left + 1; right < len(info.Constraints); right++ {
// We can't peel off a fixed tile - it must form part of the
// word.
if !info.Draws[right] {
continue
}
emit(left, right)
}
}
}()
return out
}