// Strings: UTF-16 code units, as JS counts them, and the string natives
#pragma once

#include "core.h"

string_t jsrt_string_lit(const char *utf8);
string_t jsrt_string_from_utf8(const char *utf8, size_t size);
int32_t jsrt_string_length(string_t s);
bool jsrt_string_eq(string_t a, string_t b);
int32_t jsrt_string_cmp(string_t a, string_t b);

// `s[i]`: the code unit at `i`, as a string, or JSRT_UNDEFINED past the end
string_t jsrt_string_at(string_t s, int32_t i);

#ifdef JSRT_IMPLEMENTATION
#include <stdlib.h>
#include <string.h>

struct jsrt_string {
    int32_t length;
    uint16_t units[];
};

static string_t jsrt_string_new(int32_t length) {
    string_t s = jsrt_alloc(sizeof *s + (size_t)length * sizeof(uint16_t));
    s->length = length;
    return s;
}

string_t jsrt_string_lit(const char *utf8) {
    return jsrt_string_from_utf8(utf8, strlen(utf8));
}

// The `size` bytes at `utf8`, which need not end in a 0
string_t jsrt_string_from_utf8(const char *utf8, size_t size) {
    const unsigned char *p = (const unsigned char *)utf8, *end = p + size;
    string_t s = jsrt_string_new((int32_t)size * 2);
    int32_t n = 0;

    while (p < end) {
        uint32_t c = *p++;
        int extra = c >= 0xF0 ? 3 : c >= 0xE0 ? 2 : c >= 0xC0 ? 1 : 0;
        c &= extra ? 0x3F >> extra : 0x7F;

        for (; extra && p < end; extra--) {
            c = (c << 6) | (*p++ & 0x3F);
        }

        if (c >= 0x10000) {
            c -= 0x10000;
            s->units[n++] = (uint16_t)(0xD800 + (c >> 10));
            s->units[n++] = (uint16_t)(0xDC00 + (c & 0x3FF));
        } else {
            s->units[n++] = (uint16_t)c;
        }
    }

    s->length = n;
    return s;
}

int32_t jsrt_string_length(string_t s) {
    return s->length;
}

bool jsrt_string_eq(string_t a, string_t b) {
    return a->length == b->length && !memcmp(a->units, b->units, (size_t)a->length * sizeof(uint16_t));
}

int32_t jsrt_string_cmp(string_t a, string_t b) {
    int32_t n = a->length < b->length ? a->length : b->length;

    for (int32_t i = 0; i < n; i++) {
        if (a->units[i] != b->units[i]) {
            return a->units[i] < b->units[i] ? -1 : 1;
        }
    }

    return a->length < b->length ? -1 : a->length > b->length;
}

string_t jsrt_string_at(string_t s, int32_t i) {
    if (i < 0 || i >= s->length) {
        return JSRT_UNDEFINED;
    }

    string_t c = jsrt_string_new(1);
    c->units[0] = s->units[i];
    return c;
}

// `s` as UTF-8, in a new buffer for the caller to free
static char *jsrt_string_utf8(string_t s) {
    char *utf8 = jsrt_alloc((size_t)s->length * 3 + 1);
    char *out = utf8;

    for (int32_t i = 0; i < s->length; i++) {
        uint32_t c = s->units[i];
        bool pair = c >= 0xD800 && c < 0xDC00 && i + 1 < s->length
            && s->units[i + 1] >= 0xDC00 && s->units[i + 1] < 0xE000;

        if (pair) {
            c = 0x10000 + ((c - 0xD800) << 10) + (s->units[++i] - 0xDC00);
        }

        if (c < 0x80) {
            *out++ = (char)c;
        } else if (c < 0x800) {
            *out++ = (char)(0xC0 | c >> 6);
            *out++ = (char)(0x80 | (c & 0x3F));
        } else if (c < 0x10000) {
            *out++ = (char)(0xE0 | c >> 12);
            *out++ = (char)(0x80 | (c >> 6 & 0x3F));
            *out++ = (char)(0x80 | (c & 0x3F));
        } else {
            *out++ = (char)(0xF0 | c >> 18);
            *out++ = (char)(0x80 | (c >> 12 & 0x3F));
            *out++ = (char)(0x80 | (c >> 6 & 0x3F));
            *out++ = (char)(0x80 | (c & 0x3F));
        }
    }

    *out = 0;
    return utf8;
}

// ── natives ──

string_t __native_string_concat(string_t a, string_t b) {
    string_t s = jsrt_string_new(a->length + b->length);
    memcpy(s->units, a->units, (size_t)a->length * sizeof(uint16_t));
    memcpy(s->units + a->length, b->units, (size_t)b->length * sizeof(uint16_t));
    return s;
}

string_t __native_string_slice(string_t s, int32_t start, int32_t end) {
    start = start < 0 ? 0 : start > s->length ? s->length : start;
    end = end < start ? start : end > s->length ? s->length : end;

    string_t r = jsrt_string_new(end - start);
    memcpy(r->units, s->units + start, (size_t)(end - start) * sizeof(uint16_t));
    return r;
}

int32_t __native_string_char_code_at(string_t s, int32_t i) {
    return s->units[i];
}

string_t __native_string_from_char_code(int32_t code) {
    string_t s = jsrt_string_new(1);
    s->units[0] = (uint16_t)code;
    return s;
}

#endif // JSRT_IMPLEMENTATION