arrays: add map_of_counts/1, map_of_indexes/1 as well as index_of_first/2, index_of_last/2 utilities (#16618)

vlib/arrays/map_of.v

@@ -0,0 +1,29 @@
+module arrays
+
+// map_of_indexes returns a map, where each key is an unique value in `array`,
+// and each value for that key is an array, containing the indexes in `array`,
+// where that value has been found.
+// Example: arrays.map_of_indexes([1,2,3,4,4,2,1,4,4,999]) == {1: [0, 6], 2: [1, 5], 3: [2], 4: [3, 4, 7, 8], 999: [9]}
+pub fn map_of_indexes[T](array []T) map[T][]int {
+	mut result := map[T][]int{}
+	for i, e in array {
+		if _ := result[e] {
+			result[e] << i
+		} else {
+			result[e] = [i]
+		}
+	}
+	return result
+}
+
+// map_of_counts returns a map, where each key is an unique value in `array`,
+// and each value for that key is how many times that value occures in `array`.
+// It can be useful for building histograms of discrete measurements.
+// Example: arrays.map_of_counts([1,2,3,4,4,2,1,4,4]) == {1: 2, 2: 2, 3: 1, 4: 4}
+pub fn map_of_counts[T](array []T) map[T]int {
+	mut result := map[T]int{}
+	for e in array {
+		result[e]++
+	}
+	return result
+}