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package tool
// TransformMap transforms map of type IN into a map of type OUT.
// The main point of this function is to optimize memory usage. It allocates a single slice for
// the elements in values of the map rather than a slice per key.
func TransformMap[K comparable, V1 any, V2 any, IN ~map[K]V1, OUT ~map[K]V2, E any](
in IN,
v1Len func(V1) int,
appendV1ToSlice func(V1, []E) []E,
elemToV2 func([]E) V2) OUT {
if len(in) == 0 {
return nil
}
result := make(OUT, len(in))
valsLen := 0
for _, v := range in {
valsLen += v1Len(v)
}
vals := make([]E, 0, valsLen) // allocate backing array for all elements in one go
for k, v := range in {
vals = appendV1ToSlice(v, vals)
// set capacity to length to protect against potential append overwriting next value
lk := len(vals)
result[k] = elemToV2(vals[:lk:lk])
vals = vals[lk:]
}
return result
}
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