From c3c1b8d0838dd4b1b57d75340d0bf26c22f035d6 Mon Sep 17 00:00:00 2001
From: Richard Feldman <oss@rtfeldman.com>
Date: Wed, 29 Mar 2023 15:52:08 -0400
Subject: [PATCH] Add Str.walkUtf8

---
 crates/compiler/builtins/roc/Str.roc    | 28 +++++++++++++++++++++++++
 crates/compiler/module/src/symbol.rs    |  1 +
 crates/compiler/test_gen/src/gen_str.rs | 27 ++++++++++++++++++++++++
 3 files changed, 56 insertions(+)

diff --git a/crates/compiler/builtins/roc/Str.roc b/crates/compiler/builtins/roc/Str.roc
index 6a670d71a8..f98d4360f0 100644
--- a/crates/compiler/builtins/roc/Str.roc
+++ b/crates/compiler/builtins/roc/Str.roc
@@ -107,6 +107,7 @@ interface Str
         replaceLast,
         splitFirst,
         splitLast,
+        walkUtf8,
         walkUtf8WithIndex,
         reserve,
         releaseExcessCapacity,
@@ -841,6 +842,33 @@ walkUtf8WithIndexHelp = \string, state, step, index, length ->
     else
         state
 
+## Walks over the `UTF-8` bytes of the given [Str] and calls a function to update
+## state for each byte.
+##
+## ```
+## result = walkUtf8 "hello, world!" "" (\state, byte -> state ++ String.fromCodePoint byte)
+## expect result == Ok "hello, world!"
+## ```
+walkUtf8 : Str, state, (state, U8 -> state) -> state
+walkUtf8 = \str, initial, step ->
+    walkUtf8Help str initial step 0 (Str.countUtf8Bytes str)
+
+walkUtf8Help : Str, state, (state, U8 -> state), Nat, Nat -> state
+walkUtf8Help = \str, state, step, index, length ->
+    if index < length then
+        byte = Str.getUnsafe str index
+        newState = step state byte
+
+        walkUtf8Help str newState step (index + 1) length
+    else
+        state
+
+# Test walkUtf8 with a simple ASCII string
+expect (walkUtf8 "ABC" [] List.append) == [65, 66, 67]
+
+# Test walkUtf8 with a multi-byte string
+expect (walkUtf8 "鹏" [] List.append) == [233, 185, 143]
+
 ## Shrink the memory footprint of a str such that it's capacity and length are equal.
 ## Note: This will also convert seamless slices to regular lists.
 releaseExcessCapacity : Str -> Str
diff --git a/crates/compiler/module/src/symbol.rs b/crates/compiler/module/src/symbol.rs
index 4edc38e50d..7ceaf869e9 100644
--- a/crates/compiler/module/src/symbol.rs
+++ b/crates/compiler/module/src/symbol.rs
@@ -1328,6 +1328,7 @@ define_builtins! {
         55 STR_GRAPHEMES: "graphemes"
         56 STR_IS_VALID_SCALAR: "isValidScalar"
         57 STR_RELEASE_EXCESS_CAPACITY: "releaseExcessCapacity"
+        58 STR_WALK_UTF8: "walkUtf8"
     }
     6 LIST: "List" => {
         0 LIST_LIST: "List" exposed_apply_type=true // the List.List type alias
diff --git a/crates/compiler/test_gen/src/gen_str.rs b/crates/compiler/test_gen/src/gen_str.rs
index 19b8a89c79..dacf7feea6 100644
--- a/crates/compiler/test_gen/src/gen_str.rs
+++ b/crates/compiler/test_gen/src/gen_str.rs
@@ -1822,6 +1822,33 @@ fn str_split_overlapping_substring_2() {
     );
 }
 
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-dev"))]
+fn str_walk_utf8() {
+    #[cfg(not(feature = "gen-llvm-wasm"))]
+    assert_evals_to!(
+        // Reverse the bytes
+        indoc!(
+            r#"
+            Str.walkUtf8 "abcd" [] (\list, byte -> List.prepend list byte)
+            "#
+        ),
+        RocList::from_slice(&[b'd', b'c', b'b', b'a']),
+        RocList<u8>
+    );
+
+    #[cfg(feature = "gen-llvm-wasm")]
+    assert_evals_to!(
+        indoc!(
+            r#"
+            Str.walkUtf8WithIndex "abcd" [] (\list, byte, index -> List.append list (Pair index byte))
+            "#
+        ),
+        RocList::from_slice(&[(0, 'a'), (1, 'b'), (2, 'c'), (3, 'd')]),
+        RocList<(u32, char)>
+    );
+}
+
 #[test]
 #[cfg(any(feature = "gen-llvm", feature = "gen-dev"))]
 fn str_walk_utf8_with_index() {