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/*
 * parsing/string.rs
 *
 * ftml - Library to parse Wikidot text
 * Copyright (C) 2019-2022 Wikijump Team
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 */

use std::borrow::Cow;

/// Parses a double-quoted string.
///
/// Takes inputs starting and ending with `"`
/// and containing characters, or any of these
/// escapes:
/// * `\\`
/// * `\"`
/// * `\'`
/// * `\r`
/// * `\n`
/// * `\t`
///
/// # Panics
/// Assumes that the string is in the proper form.
/// If it is not, this function may panic.
pub fn parse_string(input: &str) -> Cow<str> {
    // We could do an iteration thing, but tracking
    // the index across replacements is complicated.
    //
    // So we check if there are any escapes, and if so,
    // build a new string.

    let input = slice_middle(input);
    if !input.contains('\\') {
        return Cow::Borrowed(input);
    }

    let mut output = String::new();
    let mut wants_escape = false;

    for ch in input.chars() {
        if wants_escape {
            let replacement = match ch {
                '\\' => '\\',
                '\"' => '\"',
                '\'' => '\'',
                'r' => '\r',
                'n' => '\n',
                't' => '\t',
                _ => panic!("Invalid escape sequence: '\\{ch}'"),
            };

            output.push(replacement);
            wants_escape = false;
        } else if ch == '\\' {
            wants_escape = true;
        } else {
            output.push(ch);
        }
    }

    Cow::Owned(output)
}

/// Slices the first and last characters off of the string.
/// Assumes there are codepoint boundaries there.
fn slice_middle(input: &str) -> &str {
    let len = input.len();
    let last = len - 1;

    &input[1..last]
}

#[test]
fn test_parse_string() {
    macro_rules! test {
        ($input:expr, $expected:expr, $variant:tt $(,)?) => {{
            let actual = parse_string($input);

            assert_eq!(
                &actual, $expected,
                "Actual string (left) doesn't match expected (right)"
            );

            assert!(
                matches!(actual, Cow::$variant(_)),
                "Outputted string of the incorrect variant",
            );
        }};
    }

    test!(r#""""#, "", Borrowed);
    test!(r#""!""#, "!", Borrowed);
    test!(r#""\"""#, "\"", Owned);
    test!(r#""\'""#, "\'", Owned);
    test!(r#""apple banana""#, "apple banana", Borrowed);
    test!(r#""abc \\""#, "abc \\", Owned);
    test!(r#""\n def""#, "\n def", Owned);
    test!(
        r#""abc \t (\\\t) \r (\\\r) def""#,
        "abc \t (\\\t) \r (\\\r) def",
        Owned,
    );
}

#[test]
#[should_panic]
fn test_parse_string_escape() {
    // This shouldn't happen in real code, since
    // any invalid string constructions are caught at the
    // tokenization stage.
    //
    // However we're testing this to ensure high code coverage.

    let _ = parse_string(r#""Invalid escape! \z""#);
}

#[test]
fn test_slice_middle() {
    macro_rules! test {
        ($input:expr, $expected:expr $(,)?) => {{
            let actual = slice_middle($input);

            assert_eq!(
                actual, $expected,
                "Actual (left) doesn't match expected (right)",
            );
        }};
    }

    test!(r#""""#, "");
    test!(r#""!""#, "!");
    test!(r#""abc""#, "abc");
    test!(r#""apple banana cherry""#, "apple banana cherry");
}