// Copyright 2023 The Pigweed Authors
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may not
// use this file except in compliance with the License. You may obtain a copy of
// the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
// License for the specific language governing permissions and limitations under
// the License.

use std::{collections::HashMap, str::FromStr};

use nom::{
    branch::alt,
    bytes::complete::{is_a, is_not, tag},
    character::complete::{alpha1, alphanumeric1},
    combinator::{map, recognize},
    multi::{many0_count, many1},
    sequence::{delimited, pair},
    IResult,
};
use once_cell::sync::Lazy;
use regex::Regex;

use crate::{Error, Result};

#[derive(Debug)]
enum StringFragment {
    Literal(String),
    Variable(String),
    OpenBrace,
    CloseBrace,
}

#[derive(Debug)]
pub struct StringSub {
    fragments: Vec<StringFragment>,
}

static VARIABLE_NAME_REGEX: Lazy<Regex> =
    Lazy::new(|| Regex::new(r"^[a-zA-Z_][a-zA-Z0-9_-]*$").expect("regex is valid"));

/// Parser for a `StringSub` variable name. Starts with either a letter or
/// underscore, and can contain alphanumeric characters, underscores, or hyphens.
fn identifier(input: &str) -> IResult<&str, &str> {
    recognize(pair(
        alt((alpha1, tag("_"))),
        many0_count(alt((alphanumeric1, is_a("_-")))),
    ))(input)
}

/// Parser for a `StringSub` variable placeholder, which is an identifier
/// enclosed in braces.
fn variable_sub(input: &str) -> IResult<&str, StringFragment> {
    map(delimited(tag("{"), identifier, tag("}")), |s: &str| {
        StringFragment::Variable(s.to_owned())
    })(input)
}

/// Parser for a `StringSub` literal string, containing any characters other
/// than braces.
fn string_literal(input: &str) -> IResult<&str, StringFragment> {
    map(is_not("{}"), |s: &str| {
        StringFragment::Literal(s.to_owned())
    })(input)
}

fn escaped_open_brace(input: &str) -> IResult<&str, StringFragment> {
    map(tag("{{"), |_| StringFragment::OpenBrace)(input)
}

fn escaped_close_brace(input: &str) -> IResult<&str, StringFragment> {
    map(tag("}}"), |_| StringFragment::CloseBrace)(input)
}

impl StringSub {
    pub fn valid_variable_name(var: &str) -> bool {
        VARIABLE_NAME_REGEX.is_match(var)
    }

    pub fn new(string: &str) -> Result<Self> {
        if string.is_empty() {
            return Ok(Self {
                fragments: Vec::new(),
            });
        }

        let mut parser = many1(alt((
            escaped_open_brace,
            escaped_close_brace,
            variable_sub,
            string_literal,
        )));

        // TODO(frolv): Map the nom error to a useful user-facing error.
        let (remainder, fragments) = parser(string).map_err(|_| Error::GenericErrorPlaceholder)?;
        if !remainder.is_empty() {
            // TODO(frolv): Some of the string wasn't parsed.
            return Err(Error::GenericErrorPlaceholder);
        }

        Ok(Self { fragments })
    }

    /// Returns an iterator over the variable names in the string.
    pub fn vars(&self) -> impl Iterator<Item = &str> {
        self.fragments.iter().filter_map(|f| match f {
            StringFragment::Variable(s) => Some(s.as_str()),
            _ => None,
        })
    }

    pub fn substitute<'a>(&self, vars: &HashMap<&'a str, &'a str>) -> Result<String> {
        let mut s = String::new();

        for frag in &self.fragments {
            match frag {
                StringFragment::Literal(lit) => s.push_str(lit),
                StringFragment::Variable(var) => {
                    let Some(&value) = vars.get(var.as_str()) else {
                        // TODO(frolv): no value provided for `var`.
                        return Err(Error::GenericErrorPlaceholder);
                    };
                    s.push_str(value);
                }
                StringFragment::OpenBrace => s.push('{'),
                StringFragment::CloseBrace => s.push('}'),
            }
        }

        Ok(s)
    }
}

impl FromStr for StringSub {
    type Err = Error;

    fn from_str(s: &str) -> std::result::Result<Self, Self::Err> {
        StringSub::new(s)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn substitute_one_var() {
        let string = StringSub::new("hello, {foo}!").unwrap();
        assert_eq!(string.vars().count(), 1);
        let vars = HashMap::from([("foo", "world")]);
        assert_eq!(string.substitute(&vars).unwrap(), "hello, world!");
    }

    #[test]
    fn substitute_multiple_vars() {
        let string = StringSub::new("{greeting}, {subject}!").unwrap();
        assert_eq!(string.vars().count(), 2);
        let vars = HashMap::from([("greeting", "hello"), ("subject", "world")]);
        assert_eq!(string.substitute(&vars).unwrap(), "hello, world!");
    }

    #[test]
    fn substitute_repeated_var() {
        let string = StringSub::new("b{v}{v}a").unwrap();
        assert_eq!(string.vars().count(), 2);
        let vars = HashMap::from([("v", "an")]);
        assert_eq!(string.substitute(&vars).unwrap(), "banana");
    }

    #[test]
    fn substitute_full_string() {
        let string = StringSub::new("{foo}").unwrap();
        assert_eq!(string.vars().count(), 1);
        let vars = HashMap::from([("foo", "hello, world!")]);
        assert_eq!(string.substitute(&vars).unwrap(), "hello, world!");
    }

    #[test]
    fn substitute_no_vars() {
        let string = StringSub::new("hello").unwrap();
        assert_eq!(string.vars().count(), 0);
        let mut vars = HashMap::new();
        assert_eq!(string.substitute(&vars).unwrap(), "hello");
        vars.insert("foo", "world");
        assert_eq!(string.substitute(&vars).unwrap(), "hello");
    }

    #[test]
    fn substitute_empty_string() {
        let string = StringSub::new("").unwrap();
        assert_eq!(string.vars().count(), 0);
        let mut vars = HashMap::new();
        assert_eq!(string.substitute(&vars).unwrap(), "");
        vars.insert("foo", "world");
        assert_eq!(string.substitute(&vars).unwrap(), "");
    }

    #[test]
    fn substitute_escaped_braces() {
        let string = StringSub::new("hello, {{foo}}").unwrap();
        assert_eq!(string.vars().count(), 0);
        let mut vars = HashMap::new();
        assert_eq!(string.substitute(&vars).unwrap(), "hello, {foo}");
        vars.insert("foo", "world");
        assert_eq!(string.substitute(&vars).unwrap(), "hello, {foo}");

        assert_eq!(
            StringSub::new("{{").unwrap().substitute(&vars).unwrap(),
            "{",
        );
        assert_eq!(
            StringSub::new("}}").unwrap().substitute(&vars).unwrap(),
            "}",
        );
        assert_eq!(
            StringSub::new("{{}}").unwrap().substitute(&vars).unwrap(),
            "{}",
        );
    }

    #[test]
    fn substitute_variable_names() {
        assert!(StringSub::new("{f}").is_ok());
        assert!(StringSub::new("{foo}").is_ok());
        assert!(StringSub::new("{__foo__}").is_ok());
        assert!(StringSub::new("{foo123}").is_ok());
        assert!(StringSub::new("{FooBar}").is_ok());
        assert!(StringSub::new("{f-o-o-}").is_ok());
        assert!(StringSub::new("{123foo}").is_err());
        assert!(StringSub::new("{-foo}").is_err());
        assert!(StringSub::new("{foo#bar}").is_err());
        assert!(StringSub::new("{!%(@)}").is_err());
    }

    #[test]
    fn substitute_invalid_format_string() {
        assert!(StringSub::new("{").is_err());
        assert!(StringSub::new("}").is_err());
        assert!(StringSub::new("hello, {foo").is_err());
        assert!(StringSub::new("hello, {}").is_err());
        assert!(StringSub::new("this is a closing brace: }").is_err());
        assert!(StringSub::new("hello, {foo{").is_err());
        assert!(StringSub::new("foo{{bar}baz").is_err());
        assert!(StringSub::new("{greeting}, {subject").is_err());
    }

    #[test]
    fn substitute_missing_variables() {
        let string = StringSub::new("hello, {foo}!").unwrap();
        assert_eq!(string.vars().count(), 1);
        let vars = HashMap::new();
        assert!(string.substitute(&vars).is_err());
    }
}