// The Finch lexer, written in Finch.
// A token's kind is a word: "ident", "int", "float", "string", "char", "eof",
// or the keyword / operator itself ("fn", "{", "+=", ...).

struct Token {
    str kind
    str text
    int line
    int col
    bool nl        // a newline came before this token
}

fn isAlpha(char c) -> bool {
    return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '_'
}

fn isDigit(char c) -> bool {
    return c >= '0' && c <= '9'
}

fn isHex(char c) -> bool {
    return isDigit(c) || (c >= 'a' && c <= 'f') || (c >= 'A' && c <= 'F')
}

fn isKeyword(str w) -> bool {
    keywords := ["fn", "return", "if", "else", "while", "for", "in", "break", "continue", "true", "false",
                 "null", "import", "link", "struct", "defer", "or", "try", "enum", "match"]
    return keywords.contains(w)
}

struct Lexer {
    str src
    str path
    int i
    int line
    int col
    bool nl
}

fn fail(ptr[Lexer] lx, int line, int col, str msg) {
    print(lx.path + ":" + str(line) + ":" + str(col) + ": error: " + msg)
    exit(1)
}

fn peek(ptr[Lexer] lx, int off) -> char {
    if lx.i + off < lx.src.len {
        return lx.src[lx.i + off]
    }
    return char(0)
}

fn advance(ptr[Lexer] lx) {
    c := lx.src[lx.i]
    if c == '\n' {
        lx.line += 1
        lx.col = 1
        lx.nl = true
    } else if int(c) & 0xC0 != 0x80 {
        lx.col += 1
    }
    lx.i += 1
}

// operators, longest first
fn operator(ptr[Lexer] lx) -> str {
    twos := ["->", "<<", ">>", "<=", ">=", "==", "!=", "&&", "||", "..", ":=", "+=", "-=", "*=", "/=", "%="]
    if lx.i + 1 < lx.src.len {
        two := lx.src.sub(lx.i, lx.i + 2)
        if twos.contains(two) {
            return two
        }
    }
    ones := "(){}[],.:+-*/%&|^~<>=!"
    c := lx.src.sub(lx.i, lx.i + 1)
    if ones.contains(c) {
        return c
    }
    return ""
}

fn escape(ptr[Lexer] lx, char e) -> char {
    if e == 'n' {
        return '\n'
    }
    if e == 't' {
        return '\t'
    }
    if e == 'r' {
        return '\r'
    }
    if e == '0' {
        return char(0)
    }
    if e == '\\' || e == '"' || e == '\'' {
        return e
    }
    fail(lx, lx.line, lx.col - 1, "unknown escape \\" + str(e))
    return e
}

fn lex(str src, str path) -> []Token {
    lexer := Lexer(src: src, path: path, i: 0, line: 1, col: 1, nl: true)
    lx := addr(lexer)
    []Token out
    while lx.i < lx.src.len {
        c := lx.src[lx.i]
        if c == ' ' || c == '\t' || c == '\n' || c == '\r' {
            advance(lx)
            continue
        }
        if c == '/' && peek(lx, 1) == '/' {
            while lx.i < lx.src.len && lx.src[lx.i] != '\n' {
                advance(lx)
            }
            continue
        }
        if c == '/' && peek(lx, 1) == '*' {
            line := lx.line
            col := lx.col
            advance(lx)
            advance(lx)
            while lx.i < lx.src.len && !(lx.src[lx.i] == '*' && peek(lx, 1) == '/') {
                advance(lx)
            }
            if lx.i >= lx.src.len {
                fail(lx, line, col, "comment is never closed (missing */)")
            }
            advance(lx)
            advance(lx)
            continue
        }

        t := Token(kind: "", text: "", line: lx.line, col: lx.col, nl: lx.nl)
        lx.nl = false
        start := lx.i
        if isAlpha(c) {
            while lx.i < lx.src.len && (isAlpha(lx.src[lx.i]) || isDigit(lx.src[lx.i])) {
                advance(lx)
            }
            t.text = lx.src.sub(start, lx.i)
            t.kind = "ident"
            if isKeyword(t.text) {
                t.kind = t.text
            }
        } else if isDigit(c) {
            t.kind = "int"
            if c == '0' && (peek(lx, 1) == 'x' || peek(lx, 1) == 'X') {
                advance(lx)
                advance(lx)
                while lx.i < lx.src.len && (isHex(lx.src[lx.i]) || lx.src[lx.i] == '_') {
                    advance(lx)
                }
            } else {
                while lx.i < lx.src.len && (isDigit(lx.src[lx.i]) || lx.src[lx.i] == '_') {
                    advance(lx)
                }
                if peek(lx, 0) == '.' && isDigit(peek(lx, 1)) {
                    t.kind = "float"
                    advance(lx)
                    while lx.i < lx.src.len && (isDigit(lx.src[lx.i]) || lx.src[lx.i] == '_') {
                        advance(lx)
                    }
                }
            }
            t.text = lx.src.sub(start, lx.i).replace("_", "")
        } else if c == '"' || c == '\'' {
            quote := c
            t.kind = "string"
            if quote == '\'' {
                t.kind = "char"
            }
            advance(lx)
            []str parts
            while peek(lx, 0) != quote {
                if peek(lx, 0) == char(0) || peek(lx, 0) == '\n' {
                    if quote == '"' {
                        fail(lx, t.line, t.col, "string is never closed (missing \")")
                    }
                    fail(lx, t.line, t.col, "char is never closed (missing ')")
                }
                ch := peek(lx, 0)
                advance(lx)
                if ch == '\\' {
                    e := peek(lx, 0)
                    advance(lx)
                    ch = escape(lx, e)
                }
                parts.push(str(ch))
            }
            advance(lx)
            t.text = parts.join("")
            if quote == '\'' && t.text.len != 1 {
                fail(lx, t.line, t.col, "a char holds exactly one character, like 'a'")
            }
        } else {
            op := operator(lx)
            if op.len == 0 {
                fail(lx, t.line, t.col, "unexpected character '" + str(c) + "'")
            }
            for k in 0..op.len {
                advance(lx)
            }
            t.kind = op
            t.text = op
        }
        out.push(t)
    }
    out.push(Token(kind: "eof", text: "", line: lx.line, col: lx.col, nl: true))
    return out
}