Add string search circuit example using lib (#31)

string_search_example/.gitignore

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+target/

string_search_example/Nargo.toml

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+[package]
+name = "string_search_example"
+type = "bin"
+authors = ["cypriansakwa"]
+compiler_version = ">=1.0.0"
+
+[dependencies]
+noir_string_search = {tag = "v0.3.3", git = "https://github.com/noir-lang/noir_string_search.git"}

string_search_example/Prover.toml

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+haystack = [104, 101, 108, 108, 111, 32, 119, 111, 114, 108, 100, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 
+0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 
+0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
+haystack_len = 11
+needle = [119, 111, 114, 108, 100, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
+needle_len = 5

string_search_example/README.md

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+# Noir Substring Search Circuit
+
+This project demonstrates a robust substring search circuit using the Noir language and the [`noir_string_search`](https://github.com/noir-lang/noir_string_search) Noir library.  
+It provides a wrapper around the library's substring search to handle edge cases safely, making it suitable for use in zero-knowledge circuits.
+
+## Features
+
+- **Safe Substring Search:** Returns both a `found` boolean and the index of the first match.
+- **Edge Case Handling:** Returns `(false, 0)` if the needle is empty, longer than the haystack, or not present.
+- **Noir Library Compatibility:** Only calls the underlying library when it is safe to do so, avoiding panics/assertion failures.
+- **Comprehensive Tests:** Includes unit tests for common and edge cases.
+
+## Usage
+
+### Adding noir_string_search to your project
+
+To use `noir_string_search`, add it to your `Nargo.toml` dependencies section like this:
+
+```toml
+[dependencies]
+noir_string_search = { git = "https://github.com/noir-lang/noir_string_search" }
+```
+
+Then, import it in your Noir code:
+
+```rust
+use noir_string_search::{StringBody256, SubString32};
+```
+
+### Circuit Interface
+
+```rust
+pub fn substring_search(
+    haystack: [u8; 256],
+    haystack_len: u32,
+    needle: [u8; 32],
+    needle_len: u32,
+) -> (bool, u32)
+```
+
+- `haystack`: The byte array to search in (max length 256).
+- `haystack_len`: The actual length of the haystack.
+- `needle`: The substring to search for (max length 32).
+- `needle_len`: The actual length of the needle.
+- **Returns**: `(found, index)`  
+  - `found`: `true` if the substring was found, `false` otherwise.
+  - `index`: the starting index of the first match (0 if not found).
+
+### Example
+
+```rust
+let (found, index) = main(haystack, haystack_len, needle, needle_len);
+assert(found == true);
+assert(index == 3);
+```
+
+## Implementation
+
+The circuit only attempts the underlying library search if:
+- The needle is not empty.
+- The needle length does not exceed the haystack length.
+
+Otherwise, it returns `(false, 0)`.
+
+## Test Coverage
+
+The provided tests cover:
+- Substring at start, middle, and end of haystack.
+- Needle absent from haystack.
+- Needle longer than haystack.
+- Needle at second position.
+- Full haystack and needle match.
+
+Example test:
+
+```rust
+#[test]
+fn finds_substring_at_start() -> Field {
+    // haystack = "hello", needle = "hello"
+    let (found, index) = main(haystack, 5, needle, 5);
+    assert(found == true);
+    assert(index == 0);
+    1
+}
+```
+
+## Requirements
+
+- [Noir](https://noir-lang.org/)
+- [`noir_string_search`](https://github.com/noir-lang/noir_string_search) Noir library
+
+## License
+
+MIT or Apache-2.0

string_search_example/src/main.nr

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+use noir_string_search::{StringBody256, SubString32};
+mod test_inputs;
+/// Convert a bool to Field (1 or 0 for selector logic)
+fn bool_to_field(b: bool) -> Field {
+    if b { 1 } else { 0 }
+}
+
+/// Substring search circuit-friendly: returns (found, index)
+/// - If needle is empty, found=true at index 0.
+/// - If needle too long, found=false at index 0.
+/// - Otherwise, use library match.
+pub fn substring_search(
+    haystack: [u8; 256],
+    haystack_len: u32,
+    needle: [u8; 32],
+    needle_len: u32,
+) -> (bool, u32) {
+    assert(haystack_len <= 256);
+    assert(needle_len <= 32);
+
+    let haystack_body: StringBody256 = StringBody256::new(haystack, haystack_len);
+    let needle_body: SubString32 = SubString32::new(needle, needle_len);
+    let (found, index) = haystack_body.substring_match(needle_body);
+
+    let is_empty = needle_len == 0;
+    let too_long = needle_len > haystack_len;
+
+    // Field selectors for circuit-friendly output selection
+    let is_empty_f = bool_to_field(is_empty);
+    let too_long_f = bool_to_field(too_long);
+    let normal_case_f = 1 - is_empty_f - too_long_f;
+
+    // Use selectors to choose output
+    let result_found_f: Field = is_empty_f + normal_case_f * bool_to_field(found);
+    let result_index: u32 = (is_empty_f * 0 + too_long_f * 0 + normal_case_f * index as Field) as u32;
+
+    (result_found_f == 1, result_index)
+}
+
+// Noir entry point
+fn main(
+    haystack: [u8; 256],
+    haystack_len: u32,
+    needle: [u8; 32],
+    needle_len: u32,
+) -> pub (bool, u32) {
+    substring_search(haystack, haystack_len, needle, needle_len)
+}

string_search_example/src/test_inputs.nr

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+use crate::substring_search;
+
+/// Helper to pad array to fixed length
+fn pad_to_256(input: [u8], pad: u8) -> [u8; 256] {
+    let mut out: [u8; 256] = [0; 256];
+    let len = if input.len() > 256 { 256 } else { input.len() };
+    for i in 0..len { out[i] = input[i]; }
+    for i in len..256 { out[i] = pad; }
+    out
+}
+
+fn pad_to_32(input: [u8], pad: u8) -> [u8; 32] {
+    let mut out: [u8; 32] = [0; 32];
+    let len = if input.len() > 32 { 32 } else { input.len() };
+    for i in 0..len { out[i] = input[i]; }
+    for i in len..32 { out[i] = pad; }
+    out
+}
+
+#[test]
+fn test_exact_match() {
+    // "hello world", "world"
+    let haystack = pad_to_256([104,101,108,108,111,32,119,111,114,108,100], 0);
+    let needle = pad_to_32([119,111,114,108,100], 0);
+    let (found, index) = substring_search(haystack, 11, needle, 5);
+    assert(found == true);
+    assert(index == 6);
+}
+
+#[test]
+fn test_needle_at_start() {
+    // "abc123", "abc"
+    let haystack = pad_to_256([97, 98, 99, 49, 50, 51], 0);
+    let needle = pad_to_32([97, 98, 99], 0);
+    let (found, index) = substring_search(haystack, 6, needle, 3);
+    assert(found == true);
+    assert(index == 0);
+}
+
+#[test]
+fn test_needle_at_end() {
+    // "good morning", "ning"
+    let haystack = pad_to_256([103, 111, 111, 100, 32, 109, 111, 114, 110, 105, 110, 103], 0);
+    let needle = pad_to_32([110, 105, 110, 103], 0);
+    let (found, index) = substring_search(haystack, 12, needle, 4);
+    assert(found == true);
+    assert(index == 8);
+}
+
+#[test]
+fn test_haystack_equals_needle() {
+    let text = [104,101,108,108,111,32,119,111,114,108,100];
+    let haystack = pad_to_256(text, 0);
+    let needle = pad_to_32(text, 0);
+    let (found, index) = substring_search(haystack, 11, needle, 11);
+    assert(found == true);
+    assert(index == 0);
+}
+
+// Add more positive tests for chunk boundaries, maximal lengths, etc. as needed.