topdown: fix split(..., "")

Fixes https://github.com/open-policy-agent/opa/issues/8018

goos: darwin
goarch: arm64
pkg: github.com/open-policy-agent/opa/v1/topdown
cpu: Apple M4 Max
                                        │ main.bench  │               pr.bench               │
                                        │   sec/op    │   sec/op     vs base                 │
Split-16                                  238.7n ± 1%   254.2n ± 1%  +6.47% (p=0.000 n=10)

                                        │  main.bench  │              pr.bench               │
                                        │     B/op     │    B/op     vs base                 │
Split-16                                  384.0 ± 0%     384.0 ± 0%       ~ (p=1.000 n=10) ¹
¹ all samples are equal

                                        │  main.bench  │              pr.bench               │
                                        │  allocs/op   │ allocs/op   vs base                 │
Split-16                                  14.00 ± 0%     14.00 ± 0%       ~ (p=1.000 n=10) ¹
¹ all samples are equal

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>

v1/test/cases/testdata/v1/strings/test-strings-0903.yaml

@@ -1,6 +1,6 @@
 ---
 cases:
-  - note: "strings/split: empty string"
+  - note: "strings/split: empty text string"
     query: data.generated.p = x
     modules:
       - |
@@ -12,3 +12,21 @@ cases:
     data: {}
     want_result:
       - x: ""
+  - note: "strings/split: empty delim string"
+    query: data.test.p = x
+    modules:
+      - |
+        package test
+
+        p := split("test", "")
+    want_result:
+      - x: [t, e, s, t]
+  - note: "strings/split: empty text and delim string"
+    query: data.test.p = x
+    modules:
+      - |
+        package test
+
+        p := split("", "")
+    want_result:
+      - x: []

v1/util/performance.go

@@ -79,17 +79,10 @@ func KeysCount[K comparable, V any](m map[K]V, p func(K) bool) int {
 // SplitMap calls fn for each delim-separated part of text and returns a slice of the results.
 // Cheaper than calling fn on strings.Split(text, delim), as it avoids allocating an intermediate slice of strings.
 func SplitMap[T any](text string, delim string, fn func(string) T) []T {
-	before, after, found := strings.Cut(text, delim)
-	if !found {
-		return []T{fn(text)}
+	sl := make([]T, 0, strings.Count(text, delim)+1)
+	for s := range strings.SplitSeq(text, delim) {
+		sl = append(sl, fn(s))
 	}
-
-	sl := append(make([]T, 0, strings.Count(text, delim)+1), fn(before))
-	for found {
-		before, after, found = strings.Cut(after, delim)
-		sl = append(sl, fn(before))
-	}
-
 	return sl
 }