initial commit

.gitignore

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+/target

Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "tpstrutil"
+version = "0.1.0"

Cargo.toml

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+[package]
+name = "tpstrutil"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

src/lib.rs

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+pub fn truncate_str_to_array<const N: usize>(s: &str) -> ([u8; N], usize) {
+    let mut buf = [0u8; N];
+    let tr_len = truncated_str_len(s, N);
+    // SAFETY:
+    // `truncated_str_len` is guaranteed to return a length less than or equal to both the string
+    // length and the maximum truncated length `N`.
+    let src = unsafe { s.as_bytes().get_unchecked(..tr_len) };
+    let dst = unsafe { buf.get_unchecked_mut(..tr_len) };
+    dst.copy_from_slice(src);
+    (buf, tr_len)
+}
+
+pub fn truncated_str_len(s: &str, n: usize) -> usize {
+    let bs = s.as_bytes();
+    
+    if bs.len() <= n {
+        return bs.len();
+    }
+
+    let mut tr_len = n;
+    // Repeatedly check if the byte `bs[tr_len]` (the byte immediately after the end of the
+    // candidate truncated string) is a continutation byte. We are splitting the string at a
+    // codepoint boundary (and therefore have a valid truncated string) iff this byte is not a
+    // continuation byte. There can be a maximum of 3 consecutive continuation bytes, so if the
+    // `is_utf8_continutation` check returns true 3 times in a row then we know the next byte will
+    // not be a continutation without the need to check; therefore, we can use `take` and hopefully
+    // allow the compiler to unroll the loop. `bs.len() > n` must hold here, so the slice will
+    // never panic.
+    for b in bs[..=tr_len].iter().copied().rev().take(3) {
+        if !is_utf8_continutation(b) {
+            break;
+        }
+        // This would underflow if `tr_len == 0`, but the first byte of a utf8 string is guaranteed
+        // not to be a continuation byte so this cannot happen.
+        tr_len -= 1;
+    }
+    tr_len
+}
+
+pub fn is_utf8_continutation(b: u8) -> bool {
+    b & 0b11000000 == 0b10000000
+}
+
+#[cfg(test)]
+mod tests {
+    use std::str;
+    
+    use super::{truncated_str_len, truncate_str_to_array};
+
+    #[test]
+    fn test_truncate_str_len() {
+        assert_eq!(truncated_str_len("", 0), 0);
+        assert_eq!(truncated_str_len("", 1), 0);
+        assert_eq!(truncated_str_len("", 2), 0);
+        assert_eq!(truncated_str_len("", 3), 0);
+        assert_eq!(truncated_str_len("", usize::MAX), 0);
+        
+        assert_eq!(truncated_str_len("hi", 0), 0);
+        assert_eq!(truncated_str_len("hi", 1), 1);
+        assert_eq!(truncated_str_len("hi", 2), 2);
+        assert_eq!(truncated_str_len("hi", 3), 2);
+        assert_eq!(truncated_str_len("hi", usize::MAX), 2);
+
+        assert_eq!(truncated_str_len("日本", 0), 0);
+        assert_eq!(truncated_str_len("日本", 1), 0);
+        assert_eq!(truncated_str_len("日本", 2), 0);
+        assert_eq!(truncated_str_len("日本", 3), 3);
+        assert_eq!(truncated_str_len("日本", 4), 3);
+        assert_eq!(truncated_str_len("日本", 5), 3);
+        assert_eq!(truncated_str_len("日本", 6), 6);
+        assert_eq!(truncated_str_len("日本", 7), 6);
+        assert_eq!(truncated_str_len("日本", 8), 6);
+        assert_eq!(truncated_str_len("日本", 9), 6);
+        assert_eq!(truncated_str_len("日本", usize::MAX), 6);
+
+        assert_eq!(truncated_str_len("cafe\u{0301}s", 0), 0);
+        assert_eq!(truncated_str_len("cafe\u{0301}s", 1), 1);
+        assert_eq!(truncated_str_len("cafe\u{0301}s", 2), 2);
+        assert_eq!(truncated_str_len("cafe\u{0301}s", 3), 3);
+        assert_eq!(truncated_str_len("cafe\u{0301}s", 4), 4);
+        assert_eq!(truncated_str_len("cafe\u{0301}s", 5), 4);
+        assert_eq!(truncated_str_len("cafe\u{0301}s", 6), 6);
+        assert_eq!(truncated_str_len("cafe\u{0301}s", 7), 7);
+        assert_eq!(truncated_str_len("cafe\u{0301}s", 8), 7);
+        assert_eq!(truncated_str_len("cafe\u{0301}s", usize::MAX), 7);
+    }
+
+    #[test]
+    fn test_truncate_str_to_array() {
+        fn array_to_str<const N: usize>(x: &([u8; N], usize)) -> &str {
+            str::from_utf8(&x.0[..x.1]).unwrap()
+        }
+        
+        assert_eq!(array_to_str(&truncate_str_to_array::<0>("")), "");
+        assert_eq!(array_to_str(&truncate_str_to_array::<1>("")), "");
+        assert_eq!(array_to_str(&truncate_str_to_array::<2>("")), "");
+        assert_eq!(array_to_str(&truncate_str_to_array::<3>("")), "");
+
+        assert_eq!(array_to_str(&truncate_str_to_array::<0>("hi")), "");
+        assert_eq!(array_to_str(&truncate_str_to_array::<1>("hi")), "h");
+        assert_eq!(array_to_str(&truncate_str_to_array::<2>("hi")), "hi");
+        assert_eq!(array_to_str(&truncate_str_to_array::<3>("hi")), "hi");
+
+        assert_eq!(array_to_str(&truncate_str_to_array::<0>("日本")), "");
+        assert_eq!(array_to_str(&truncate_str_to_array::<1>("日本")), "");
+        assert_eq!(array_to_str(&truncate_str_to_array::<2>("日本")), "");
+        assert_eq!(array_to_str(&truncate_str_to_array::<3>("日本")), "日");
+        assert_eq!(array_to_str(&truncate_str_to_array::<4>("日本")), "日");
+        assert_eq!(array_to_str(&truncate_str_to_array::<5>("日本")), "日");
+        assert_eq!(array_to_str(&truncate_str_to_array::<6>("日本")), "日本");
+        assert_eq!(array_to_str(&truncate_str_to_array::<7>("日本")), "日本");
+
+        assert_eq!(array_to_str(&truncate_str_to_array::<0>("cafe\u{0301}s")), "");
+        assert_eq!(array_to_str(&truncate_str_to_array::<1>("cafe\u{0301}s")), "c");
+        assert_eq!(array_to_str(&truncate_str_to_array::<2>("cafe\u{0301}s")), "ca");
+        assert_eq!(array_to_str(&truncate_str_to_array::<3>("cafe\u{0301}s")), "caf");
+        assert_eq!(array_to_str(&truncate_str_to_array::<4>("cafe\u{0301}s")), "cafe");
+        assert_eq!(array_to_str(&truncate_str_to_array::<5>("cafe\u{0301}s")), "cafe");
+        assert_eq!(array_to_str(&truncate_str_to_array::<6>("cafe\u{0301}s")), "cafe\u{0301}");
+        assert_eq!(array_to_str(&truncate_str_to_array::<7>("cafe\u{0301}s")), "cafe\u{0301}s");
+        assert_eq!(array_to_str(&truncate_str_to_array::<8>("cafe\u{0301}s")), "cafe\u{0301}s");
+    }
+}
+