// runtime/string.el — String operations implemented in El.
//
// All functions delegate character-level work to the seed primitives declared
// in el_seed.c.  No C is written here; this is pure El source that compiles
// to C via the normal El pipeline.
//
// Seed primitives used (provided by el_seed.c):
//   __str_len(s)               -> Int
//   __str_char_at(s, i)        -> Int   (char code at byte index i)
//   __str_alloc(n)             -> String (n-byte zero-filled mutable buffer)
//   __str_set_char(s, i, c)    -> String (mutate s[i]=c, return s)
//   __str_cmp(a, b)            -> Int   (strcmp)
//   __str_ncmp(a, b, n)        -> Int   (strncmp)
//   __str_concat_raw(a, b)     -> String
//   __str_slice_raw(s, lo, hi) -> String (substring copy [lo, hi))
//   __int_to_str(n)            -> String
//   __str_to_int(s)            -> Int
//   __float_to_str(f)          -> String
//   __str_to_float(s)          -> Float
//   __println(s)
//   __print(s)
//   __readline()               -> String
//   __url_encode(s)            -> String
//   __url_decode(s)            -> String

// ── I/O ──────────────────────────────────────────────────────────────────────

fn println(s: String) -> Void {
    __println(s)
}

fn print(s: String) -> Void {
    __print(s)
}

fn readline() -> String {
    return __readline()
}

// ── Type conversions ──────────────────────────────────────────────────────────

fn int_to_str(n: Int) -> String {
    return __int_to_str(n)
}

fn str_to_int(s: String) -> Int {
    return __str_to_int(s)
}

fn float_to_str(f: Float) -> String {
    return __float_to_str(f)
}

fn str_to_float(s: String) -> Float {
    return __str_to_float(s)
}

fn bool_to_str(b: Bool) -> String {
    if b { return "true" }
    return "false"
}

// ── URL encoding ──────────────────────────────────────────────────────────────

fn url_encode(s: String) -> String {
    return __url_encode(s)
}

fn url_decode(s: String) -> String {
    return __url_decode(s)
}

// ── Math ──────────────────────────────────────────────────────────────────────

fn el_abs(n: Int) -> Int {
    if n < 0 { return 0 - n }
    return n
}

fn el_max(a: Int, b: Int) -> Int {
    if a > b { return a }
    return b
}

fn el_min(a: Int, b: Int) -> Int {
    if a < b { return a }
    return b
}

// ── Core string primitives ────────────────────────────────────────────────────

fn str_len(s: String) -> Int {
    return __str_len(s)
}

fn str_eq(a: String, b: String) -> Bool {
    return __str_cmp(a, b) == 0
}

fn str_concat(a: String, b: String) -> String {
    return __str_concat_raw(a, b)
}

fn str_slice(s: String, start: Int, end: Int) -> String {
    let slen: Int = __str_len(s)
    let lo: Int = start
    if lo < 0 { lo = 0 }
    if lo > slen { lo = slen }
    let hi: Int = end
    if hi < lo { hi = lo }
    if hi > slen { hi = slen }
    return __str_slice_raw(s, lo, hi)
}

// ── Whitespace helpers (internal) ─────────────────────────────────────────────
//
// _is_ws: returns true for ASCII whitespace (space, tab, \n, \r, \f, \v).

fn _is_ws(c: Int) -> Bool {
    if c == 32 { return true }   // space
    if c == 9  { return true }   // tab
    if c == 10 { return true }   // \n
    if c == 13 { return true }   // \r
    if c == 12 { return true }   // \f
    if c == 11 { return true }   // \v
    return false
}

// Scan forward from index 0; return index of first byte not in whitespace,
// or n if the entire string is whitespace.
fn _find_first_non_ws(s: String, n: Int) -> Int {
    let i: Int = 0
    while i < n {
        if !_is_ws(__str_char_at(s, i)) { return i }
        i = i + 1
    }
    return n
}

// Scan backward from index n-1; return index of last non-whitespace byte,
// or -1 if the entire string is whitespace.
fn _find_last_non_ws(s: String, n: Int) -> Int {
    let i: Int = n - 1
    while i >= 0 {
        if !_is_ws(__str_char_at(s, i)) { return i }
        i = i - 1
    }
    return -1
}

// ── Comparison and search ─────────────────────────────────────────────────────

fn str_starts_with(s: String, prefix: String) -> Bool {
    let plen: Int = __str_len(prefix)
    let slen: Int = __str_len(s)
    if plen > slen { return false }
    return __str_ncmp(s, prefix, plen) == 0
}

fn str_ends_with(s: String, suffix: String) -> Bool {
    let slen: Int = __str_len(s)
    let suflen: Int = __str_len(suffix)
    if suflen > slen { return false }
    let tail: String = __str_slice_raw(s, slen - suflen, slen)
    return __str_cmp(tail, suffix) == 0
}

fn str_contains(s: String, sub: String) -> Bool {
    let slen: Int = __str_len(s)
    let sublen: Int = __str_len(sub)
    if sublen == 0 { return true }
    if sublen > slen { return false }
    let limit: Int = slen - sublen
    let i: Int = 0
    while i <= limit {
        let window: String = __str_slice_raw(s, i, i + sublen)
        if __str_cmp(window, sub) == 0 { return true }
        i = i + 1
    }
    return false
}

fn str_index_of(s: String, sub: String) -> Int {
    let slen: Int = __str_len(s)
    let sublen: Int = __str_len(sub)
    if sublen == 0 { return 0 }
    if sublen > slen { return -1 }
    let limit: Int = slen - sublen
    let i: Int = 0
    while i <= limit {
        let window: String = __str_slice_raw(s, i, i + sublen)
        if __str_cmp(window, sub) == 0 { return i }
        i = i + 1
    }
    return -1
}

fn str_last_index_of(s: String, sub: String) -> Int {
    let slen: Int = __str_len(s)
    let sublen: Int = __str_len(sub)
    if sublen == 0 { return slen }
    if sublen > slen { return -1 }
    let last: Int = -1
    let limit: Int = slen - sublen
    let i: Int = 0
    while i <= limit {
        let window: String = __str_slice_raw(s, i, i + sublen)
        if __str_cmp(window, sub) == 0 {
            last = i
            i = i + sublen
        } else {
            i = i + 1
        }
    }
    return last
}

fn str_index_of_all(s: String, sub: String) -> [Int] {
    let result: [Int] = el_list_empty()
    let slen: Int = __str_len(s)
    let sublen: Int = __str_len(sub)
    if sublen == 0 { return result }
    if sublen > slen { return result }
    let limit: Int = slen - sublen
    let i: Int = 0
    while i <= limit {
        let window: String = __str_slice_raw(s, i, i + sublen)
        if __str_cmp(window, sub) == 0 {
            result = el_list_append(result, i)
            i = i + sublen
        } else {
            i = i + 1
        }
    }
    return result
}

// Return the byte index of the first character in s that appears in any_of,
// or -1 if none found.
fn str_find_chars(s: String, any_of: String) -> Int {
    let slen: Int = __str_len(s)
    let alen: Int = __str_len(any_of)
    if alen == 0 { return -1 }
    let i: Int = 0
    while i < slen {
        let c: Int = __str_char_at(s, i)
        let j: Int = 0
        while j < alen {
            if c == __str_char_at(any_of, j) { return i }
            j = j + 1
        }
        i = i + 1
    }
    return -1
}

// ── Character access ──────────────────────────────────────────────────────────

// Return a one-character string at byte index i, or "" if out of range.
fn str_char_at(s: String, i: Int) -> String {
    let slen: Int = __str_len(s)
    if i < 0    { return "" }
    if i >= slen { return "" }
    return __str_slice_raw(s, i, i + 1)
}

// Return the char code (byte value) at byte index i, or 0 if out of range.
fn str_char_code(s: String, i: Int) -> Int {
    let slen: Int = __str_len(s)
    if i < 0    { return 0 }
    if i >= slen { return 0 }
    return __str_char_at(s, i)
}

// ── Case conversion ───────────────────────────────────────────────────────────

fn str_to_upper(s: String) -> String {
    let n: Int = __str_len(s)
    if n == 0 { return "" }
    let out: String = __str_alloc(n)
    let i: Int = 0
    while i < n {
        let c: Int = __str_char_at(s, i)
        // a-z (97-122) -> A-Z (65-90): subtract 32
        if c >= 97 {
            if c <= 122 { c = c - 32 }
        }
        out = __str_set_char(out, i, c)
        i = i + 1
    }
    return out
}

fn str_to_lower(s: String) -> String {
    let n: Int = __str_len(s)
    if n == 0 { return "" }
    let out: String = __str_alloc(n)
    let i: Int = 0
    while i < n {
        let c: Int = __str_char_at(s, i)
        // A-Z (65-90) -> a-z (97-122): add 32
        if c >= 65 {
            if c <= 90 { c = c + 32 }
        }
        out = __str_set_char(out, i, c)
        i = i + 1
    }
    return out
}

// Aliases used in existing El codebases.
fn str_lower(s: String) -> String {
    return str_to_lower(s)
}

fn str_upper(s: String) -> String {
    return str_to_upper(s)
}

// ── Whitespace trimming ───────────────────────────────────────────────────────

fn str_trim(s: String) -> String {
    let n: Int = __str_len(s)
    if n == 0 { return "" }
    let lo: Int = _find_first_non_ws(s, n)
    if lo == n { return "" }
    let hi: Int = _find_last_non_ws(s, n)
    return __str_slice_raw(s, lo, hi + 1)
}

fn str_lstrip(s: String) -> String {
    let n: Int = __str_len(s)
    if n == 0 { return "" }
    let lo: Int = _find_first_non_ws(s, n)
    if lo == n { return "" }
    return __str_slice_raw(s, lo, n)
}

fn str_rstrip(s: String) -> String {
    let n: Int = __str_len(s)
    if n == 0 { return "" }
    let hi: Int = _find_last_non_ws(s, n)
    if hi < 0 { return "" }
    return __str_slice_raw(s, 0, hi + 1)
}

// ── Replacement ───────────────────────────────────────────────────────────────

fn str_replace(s: String, from: String, to: String) -> String {
    let slen: Int = __str_len(s)
    let flen: Int = __str_len(from)
    if flen == 0 { return s }
    if slen == 0 { return s }
    // Scan s left-to-right; emit `to` on each match, otherwise emit one byte.
    let result: String = ""
    let i: Int = 0
    while i < slen {
        // Try to match `from` at position i
        if i + flen <= slen {
            let window: String = __str_slice_raw(s, i, i + flen)
            if __str_cmp(window, from) == 0 {
                result = __str_concat_raw(result, to)
                i = i + flen
            } else {
                let ch: String = __str_slice_raw(s, i, i + 1)
                result = __str_concat_raw(result, ch)
                i = i + 1
            }
        } else {
            // Not enough bytes left for a match — emit remainder and stop.
            let tail: String = __str_slice_raw(s, i, slen)
            result = __str_concat_raw(result, tail)
            i = slen
        }
    }
    return result
}

// ── Repetition and reversal ───────────────────────────────────────────────────

fn str_repeat(s: String, n: Int) -> String {
    if n <= 0 { return "" }
    let slen: Int = __str_len(s)
    if slen == 0 { return "" }
    let result: String = ""
    let i: Int = 0
    while i < n {
        result = __str_concat_raw(result, s)
        i = i + 1
    }
    return result
}

// Byte-reverse (correct for ASCII; for multi-byte UTF-8 codepoints this
// reverses bytes within a codepoint, which is intentional at this tier —
// Phase 2 will add grapheme-aware reversal).
fn str_reverse(s: String) -> String {
    let n: Int = __str_len(s)
    if n == 0 { return "" }
    let out: String = __str_alloc(n)
    let i: Int = 0
    while i < n {
        let c: Int = __str_char_at(s, i)
        out = __str_set_char(out, n - 1 - i, c)
        i = i + 1
    }
    return out
}

// ── Prefix/suffix stripping ───────────────────────────────────────────────────

fn str_strip_prefix(s: String, prefix: String) -> String {
    let slen: Int = __str_len(s)
    let plen: Int = __str_len(prefix)
    if plen == 0 { return s }
    if plen > slen { return s }
    if __str_ncmp(s, prefix, plen) == 0 {
        return __str_slice_raw(s, plen, slen)
    }
    return s
}

fn str_strip_suffix(s: String, suffix: String) -> String {
    let slen: Int = __str_len(s)
    let suflen: Int = __str_len(suffix)
    if suflen == 0 { return s }
    if suflen > slen { return s }
    let tail: String = __str_slice_raw(s, slen - suflen, slen)
    if __str_cmp(tail, suffix) == 0 {
        return __str_slice_raw(s, 0, slen - suflen)
    }
    return s
}

// Strip leading and trailing bytes whose char code appears in `chars`.
fn str_strip_chars(s: String, chars: String) -> String {
    let slen: Int = __str_len(s)
    let clen: Int = __str_len(chars)
    if slen == 0 { return "" }
    if clen == 0 { return s }
    let lo: Int = _find_first_not_in_charset(s, chars, slen, clen)
    if lo == slen { return "" }
    let hi: Int = _find_last_not_in_charset(s, chars, slen, clen)
    return __str_slice_raw(s, lo, hi + 1)
}

// Internal: true if char code `c` is present in the charset string.
fn _char_in_set(c: Int, chars: String, clen: Int) -> Bool {
    let j: Int = 0
    while j < clen {
        if c == __str_char_at(chars, j) { return true }
        j = j + 1
    }
    return false
}

fn _find_first_not_in_charset(s: String, chars: String, slen: Int, clen: Int) -> Int {
    let i: Int = 0
    while i < slen {
        if !_char_in_set(__str_char_at(s, i), chars, clen) { return i }
        i = i + 1
    }
    return slen
}

fn _find_last_not_in_charset(s: String, chars: String, slen: Int, clen: Int) -> Int {
    let i: Int = slen - 1
    while i >= 0 {
        if !_char_in_set(__str_char_at(s, i), chars, clen) { return i }
        i = i - 1
    }
    return -1
}

// ── Padding ───────────────────────────────────────────────────────────────────

// Pad s on the left to `width` total chars, repeating `pad` cyclically.
fn str_pad_left(s: String, width: Int, pad: String) -> String {
    let slen: Int = __str_len(s)
    if slen >= width { return s }
    let plen: Int = __str_len(pad)
    if plen == 0 { return s }
    let need: Int = width - slen
    let prefix: String = ""
    let i: Int = 0
    while i < need {
        // Select pad character at position (i mod plen)
        let pad_idx: Int = i - (i / plen) * plen
        let pc: String = __str_slice_raw(pad, pad_idx, pad_idx + 1)
        prefix = __str_concat_raw(prefix, pc)
        i = i + 1
    }
    return __str_concat_raw(prefix, s)
}

// Pad s on the right to `width` total chars, repeating `pad` cyclically.
fn str_pad_right(s: String, width: Int, pad: String) -> String {
    let slen: Int = __str_len(s)
    if slen >= width { return s }
    let plen: Int = __str_len(pad)
    if plen == 0 { return s }
    let need: Int = width - slen
    let suffix: String = ""
    let i: Int = 0
    while i < need {
        let pad_idx: Int = i - (i / plen) * plen
        let pc: String = __str_slice_raw(pad, pad_idx, pad_idx + 1)
        suffix = __str_concat_raw(suffix, pc)
        i = i + 1
    }
    return __str_concat_raw(s, suffix)
}

// ── Counting ──────────────────────────────────────────────────────────────────

// Count non-overlapping occurrences of `sub` in `s`. Empty sub returns 0.
fn str_count(s: String, sub: String) -> Int {
    let slen: Int = __str_len(s)
    let sublen: Int = __str_len(sub)
    if sublen == 0 { return 0 }
    if sublen > slen { return 0 }
    let count: Int = 0
    let limit: Int = slen - sublen
    let i: Int = 0
    while i <= limit {
        let window: String = __str_slice_raw(s, i, i + sublen)
        if __str_cmp(window, sub) == 0 {
            count = count + 1
            i = i + sublen
        } else {
            i = i + 1
        }
    }
    return count
}

// Byte count — alias of str_len.
fn str_count_bytes(s: String) -> Int {
    return __str_len(s)
}

// UTF-8 codepoint count: count bytes that are NOT continuation bytes (10xxxxxx).
// Continuation bytes have the pattern 10xxxxxx = 0x80..0xBF (128..191).
fn str_count_chars(s: String) -> Int {
    let n: Int = __str_len(s)
    let count: Int = 0
    let i: Int = 0
    while i < n {
        let c: Int = __str_char_at(s, i)
        // Continuation bytes are in range [128, 191]; skip them.
        // All other bytes (< 128 ASCII, or >= 192 leading bytes) start a codepoint.
        if c < 128 {
            count = count + 1
        } else {
            if c >= 192 { count = count + 1 }
        }
        i = i + 1
    }
    return count
}

// Count newline-delimited lines. A trailing newline does NOT add an extra empty line.
fn str_count_lines(s: String) -> Int {
    let n: Int = __str_len(s)
    if n == 0 { return 0 }
    let count: Int = 0
    let has_content: Bool = false
    let i: Int = 0
    while i < n {
        let c: Int = __str_char_at(s, i)
        has_content = true
        if c == 10 {  // \n
            count = count + 1
            has_content = false
        }
        i = i + 1
    }
    if has_content { count = count + 1 }
    return count
}

// Count whitespace-delimited words (non-empty tokens).
fn str_count_words(s: String) -> Int {
    let n: Int = __str_len(s)
    let count: Int = 0
    let in_word: Bool = false
    let i: Int = 0
    while i < n {
        let c: Int = __str_char_at(s, i)
        if _is_ws(c) {
            in_word = false
        } else {
            if !in_word {
                in_word = true
                count = count + 1
            }
        }
        i = i + 1
    }
    return count
}

// Count ASCII letters [A-Za-z].
fn str_count_letters(s: String) -> Int {
    let n: Int = __str_len(s)
    let count: Int = 0
    let i: Int = 0
    while i < n {
        let c: Int = __str_char_at(s, i)
        if c >= 65 {
            if c <= 90 { count = count + 1 }    // A-Z
        }
        if c >= 97 {
            if c <= 122 { count = count + 1 }   // a-z
        }
        i = i + 1
    }
    return count
}

// Count ASCII decimal digits [0-9].
fn str_count_digits(s: String) -> Int {
    let n: Int = __str_len(s)
    let count: Int = 0
    let i: Int = 0
    while i < n {
        let c: Int = __str_char_at(s, i)
        if c >= 48 {
            if c <= 57 { count = count + 1 }   // '0'-'9'
        }
        i = i + 1
    }
    return count
}

// ── Character classification ──────────────────────────────────────────────────
//
// For all predicates: empty string -> false.
// Multi-char string: ALL bytes must satisfy the predicate.

fn is_letter(s: String) -> Bool {
    let n: Int = __str_len(s)
    if n == 0 { return false }
    let i: Int = 0
    while i < n {
        let c: Int = __str_char_at(s, i)
        let ok: Bool = false
        if c >= 65 { if c <= 90  { ok = true } }   // A-Z
        if c >= 97 { if c <= 122 { ok = true } }   // a-z
        if !ok { return false }
        i = i + 1
    }
    return true
}

fn is_digit(s: String) -> Bool {
    let n: Int = __str_len(s)
    if n == 0 { return false }
    let i: Int = 0
    while i < n {
        let c: Int = __str_char_at(s, i)
        if c < 48 { return false }   // '0'
        if c > 57 { return false }   // '9'
        i = i + 1
    }
    return true
}

fn is_alphanumeric(s: String) -> Bool {
    let n: Int = __str_len(s)
    if n == 0 { return false }
    let i: Int = 0
    while i < n {
        let c: Int = __str_char_at(s, i)
        let ok: Bool = false
        if c >= 48 { if c <= 57  { ok = true } }   // 0-9
        if c >= 65 { if c <= 90  { ok = true } }   // A-Z
        if c >= 97 { if c <= 122 { ok = true } }   // a-z
        if !ok { return false }
        i = i + 1
    }
    return true
}

fn is_whitespace(s: String) -> Bool {
    let n: Int = __str_len(s)
    if n == 0 { return false }
    let i: Int = 0
    while i < n {
        if !_is_ws(__str_char_at(s, i)) { return false }
        i = i + 1
    }
    return true
}

// ASCII punctuation: 33-47, 58-64, 91-96, 123-126.
fn is_punctuation(s: String) -> Bool {
    let n: Int = __str_len(s)
    if n == 0 { return false }
    let i: Int = 0
    while i < n {
        let c: Int = __str_char_at(s, i)
        let ok: Bool = false
        if c >= 33  { if c <= 47  { ok = true } }
        if c >= 58  { if c <= 64  { ok = true } }
        if c >= 91  { if c <= 96  { ok = true } }
        if c >= 123 { if c <= 126 { ok = true } }
        if !ok { return false }
        i = i + 1
    }
    return true
}

fn is_uppercase(s: String) -> Bool {
    let n: Int = __str_len(s)
    if n == 0 { return false }
    let i: Int = 0
    while i < n {
        let c: Int = __str_char_at(s, i)
        if c < 65 { return false }   // 'A'
        if c > 90 { return false }   // 'Z'
        i = i + 1
    }
    return true
}

fn is_lowercase(s: String) -> Bool {
    let n: Int = __str_len(s)
    if n == 0 { return false }
    let i: Int = 0
    while i < n {
        let c: Int = __str_char_at(s, i)
        if c < 97  { return false }   // 'a'
        if c > 122 { return false }   // 'z'
        i = i + 1
    }
    return true
}

// ── Splitting ─────────────────────────────────────────────────────────────────

fn str_split(s: String, sep: String) -> [String] {
    let result: [String] = el_list_empty()
    let slen: Int = __str_len(s)
    let seplen: Int = __str_len(sep)
    // Empty separator: return the whole string as a single element.
    if seplen == 0 {
        result = el_list_append(result, s)
        return result
    }
    let part_start: Int = 0
    let i: Int = 0
    while i < slen {
        if i + seplen <= slen {
            let window: String = __str_slice_raw(s, i, i + seplen)
            if __str_cmp(window, sep) == 0 {
                let part: String = __str_slice_raw(s, part_start, i)
                result = el_list_append(result, part)
                i = i + seplen
                part_start = i
            } else {
                i = i + 1
            }
        } else {
            i = i + 1
        }
    }
    // Append remaining tail (may be empty string if s ended with sep).
    let tail: String = __str_slice_raw(s, part_start, slen)
    result = el_list_append(result, tail)
    return result
}

// Split into at most n parts. The nth part (index n-1) contains the remainder
// verbatim, including any further separators.  n <= 0 returns [].  n == 1
// returns [s].
fn str_split_n(s: String, sep: String, n: Int) -> [String] {
    let result: [String] = el_list_empty()
    if n <= 0 { return result }
    if n == 1 {
        result = el_list_append(result, s)
        return result
    }
    let slen: Int = __str_len(s)
    let seplen: Int = __str_len(sep)
    if seplen == 0 {
        result = el_list_append(result, s)
        return result
    }
    let part_start: Int = 0
    let parts: Int = 0
    let i: Int = 0
    while i < slen {
        if parts >= n - 1 {
            // Reached the split limit — stop splitting, emit the rest below.
            i = slen
        } else {
            if i + seplen <= slen {
                let window: String = __str_slice_raw(s, i, i + seplen)
                if __str_cmp(window, sep) == 0 {
                    let part: String = __str_slice_raw(s, part_start, i)
                    result = el_list_append(result, part)
                    i = i + seplen
                    part_start = i
                    parts = parts + 1
                } else {
                    i = i + 1
                }
            } else {
                i = i + 1
            }
        }
    }
    // Remainder verbatim.
    let tail: String = __str_slice_raw(s, part_start, slen)
    result = el_list_append(result, tail)
    return result
}

// Split on newlines. \r\n is folded to \n. Trailing empty line after a
// final \n is dropped — so "a\nb\n" yields ["a", "b"], not ["a", "b", ""].
fn str_split_lines(s: String) -> [String] {
    let result: [String] = el_list_empty()
    let n: Int = __str_len(s)
    if n == 0 { return result }
    let line_start: Int = 0
    let i: Int = 0
    while i < n {
        let c: Int = __str_char_at(s, i)
        if c == 10 {  // \n
            let lend: Int = i
            // Fold \r\n: if the byte before \n is \r, exclude it.
            if lend > line_start {
                if __str_char_at(s, lend - 1) == 13 { lend = lend - 1 }
            }
            let line: String = __str_slice_raw(s, line_start, lend)
            result = el_list_append(result, line)
            line_start = i + 1
        }
        i = i + 1
    }
    // Trailing content with no terminating \n.
    if line_start < n {
        let line: String = __str_slice_raw(s, line_start, n)
        result = el_list_append(result, line)
    }
    return result
}

// Split into a list of one-byte strings (byte-level chars).
fn str_split_chars(s: String) -> [String] {
    let result: [String] = el_list_empty()
    let n: Int = __str_len(s)
    let i: Int = 0
    while i < n {
        let ch: String = __str_slice_raw(s, i, i + 1)
        result = el_list_append(result, ch)
        i = i + 1
    }
    return result
}

// ── Joining ───────────────────────────────────────────────────────────────────

// Join a list of strings with a separator between consecutive elements.
// Empty list yields "".  Non-string elements should not be passed here.
fn str_join(parts: [String], sep: String) -> String {
    let n: Int = el_list_len(parts)
    if n == 0 { return "" }
    let result: String = el_list_get(parts, 0)
    let i: Int = 1
    while i < n {
        result = __str_concat_raw(result, sep)
        result = __str_concat_raw(result, el_list_get(parts, i))
        i = i + 1
    }
    return result
}

// ── DHARMA byte encoding (str_to_bytes) ──────────────────────────────────────
//
// str_to_bytes — encode a string as a JSON array of unsigned byte values.
// "hi" -> "[104,105]"
// Used by db.el to store content in Engram JSON nodes as a byte array.
// Note: bytes_to_str (the inverse) is defined in json.el because it depends
// on json_array_get_string which is defined there.
fn str_to_bytes(s: String) -> String {
    let n: Int = __str_len(s)
    if n == 0 { return "[]" }
    let result: String = "["
    let i: Int = 0
    while i < n {
        let b: Int = __str_char_at(s, i)
        if i > 0 { result = __str_concat_raw(result, ",") }
        result = __str_concat_raw(result, __int_to_str(b))
        i = i + 1
    }
    return __str_concat_raw(result, "]")
}

// ── Cryptographic hashing ─────────────────────────────────────────────────────

// hash_sha256 — return the SHA-256 hex digest of a string.
// Delegates to the __sha256_hex seed primitive.
fn hash_sha256(s: String) -> String {
    return __sha256_hex(s)
}