use std::collections::HashSet;
use std::env;
use std::fs;

const SHELL_BUILTINS: &[&str] = &[
    ".", ":", "alias", "bg", "bind", "break", "builtin", "caller", "cd",
    "command", "compgen", "complete", "compopt", "continue", "declare",
    "dirs", "disown", "echo", "enable", "eval", "exec", "exit", "export",
    "false", "fc", "fg", "getopts", "hash", "help", "history", "jobs",
    "kill", "let", "local", "logout", "mapfile", "popd", "printf", "pushd",
    "pwd", "read", "readarray", "readonly", "return", "set", "shift",
    "shopt", "source", "suspend", "test", "times", "trap", "true", "type",
    "typeset", "ulimit", "umask", "unalias", "unset", "wait",
];

/// Cached command set for fast lookup without fork.
pub struct CommandCache {
    commands: HashSet<String>,
}

impl CommandCache {
    pub fn new() -> Self {
        let mut commands = HashSet::new();

        // Shell builtins
        for &b in SHELL_BUILTINS {
            commands.insert(b.to_string());
        }

        // Scan PATH directories
        if let Ok(path_var) = env::var("PATH") {
            for dir in path_var.split(':') {
                if let Ok(entries) = fs::read_dir(dir) {
                    for entry in entries.flatten() {
                        if let Some(name) = entry.file_name().to_str() {
                            commands.insert(name.to_string());
                        }
                    }
                }
            }
        }

        Self { commands }
    }

    pub fn contains(&self, cmd: &str) -> bool {
        self.commands.contains(cmd)
    }
}

/// Determine whether user input should be executed as a shell command.
pub fn is_command(input: &str, cache: &CommandCache) -> bool {
    let trimmed = input.trim();
    if trimmed.is_empty() {
        return false;
    }

    // Shell operators: pipe, redirect, background, semicolon, logical operators
    if contains_shell_operator(trimmed) {
        return true;
    }

    // Variable assignment: FOO=bar
    if is_variable_assignment(trimmed) {
        return true;
    }

    // Extract first token
    let first_token = match trimmed.split_whitespace().next() {
        Some(t) => t,
        None => return false,
    };

    // Lookup in cached command set (no fork)
    cache.contains(first_token)
}

fn contains_shell_operator(input: &str) -> bool {
    let mut in_single = false;
    let mut in_double = false;
    let mut prev = '\0';

    for ch in input.chars() {
        match ch {
            '\'' if !in_double && prev != '\\' => in_single = !in_single,
            '"' if !in_single && prev != '\\' => in_double = !in_double,
            '|' | ';' if !in_single && !in_double => return true,
            '>' | '<' if !in_single && !in_double => return true,
            '&' if !in_single && !in_double => return true,
            _ => {}
        }
        prev = ch;
    }
    false
}

fn is_variable_assignment(input: &str) -> bool {
    if let Some(eq_pos) = input.find('=') {
        if eq_pos == 0 {
            return false;
        }
        let name = &input[..eq_pos];
        if name.contains(' ') {
            return false;
        }
        let first = name.chars().next().unwrap();
        if !first.is_alphabetic() && first != '_' {
            return false;
        }
        return name.chars().all(|c| c.is_alphanumeric() || c == '_');
    }
    false
}

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

    fn cache() -> CommandCache {
        CommandCache::new()
    }

    #[test]
    fn test_builtins() {
        let c = cache();
        assert!(is_command("cd /tmp", &c));
        assert!(is_command("export FOO=bar", &c));
        assert!(is_command("echo hello", &c));
        assert!(is_command("pwd", &c));
    }

    #[test]
    fn test_path_commands() {
        let c = cache();
        assert!(is_command("ls", &c));
        assert!(is_command("ls -la", &c));
        assert!(is_command("cat /etc/hostname", &c));
    }

    #[test]
    fn test_shell_operators() {
        let c = cache();
        assert!(is_command("ls | grep foo", &c));
        assert!(is_command("echo hello > /tmp/out", &c));
        assert!(is_command("cat file1 && cat file2", &c));
    }

    #[test]
    fn test_variable_assignment() {
        let c = cache();
        assert!(is_command("FOO=bar", &c));
        assert!(is_command("MY_VAR=hello", &c));
    }

    #[test]
    fn test_ai_input() {
        let c = cache();
        assert!(!is_command("macbookがフリーズする原因を調べて", &c));
        assert!(!is_command("hello world what is this", &c));
        assert!(!is_command("Rustでhello worldを書いて", &c));
    }

    #[test]
    fn test_empty() {
        let c = cache();
        assert!(!is_command("", &c));
        assert!(!is_command("   ", &c));
    }
}