use std::collections::BTreeMap;
use std::fmt;

use serde::{Deserialize, Serialize};

pub const MAX_VALUE_DEPTH: usize = 32;
pub const MAX_VALUE_NODES: usize = 4096;
pub const MAX_VALUE_BYTES: usize = 1024 * 1024;

#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Serialize, Deserialize)]
#[serde(try_from = "String", into = "String")]
pub struct ActorRef([u8; 21]);

impl ActorRef {
    pub fn new() -> Self {
        Self::parse(&nanoid::nanoid!()).expect("Nano ID uses the supported alphabet")
    }

    pub fn as_str(&self) -> &str {
        std::str::from_utf8(&self.0).expect("Nano ID is valid UTF-8")
    }

    pub fn parse(value: &str) -> Option<Self> {
        if value.len() != 21
            || !value
                .bytes()
                .all(|byte| byte.is_ascii_alphanumeric() || matches!(byte, b'_' | b'-'))
        {
            return None;
        }
        let mut bytes = [0; 21];
        bytes.copy_from_slice(value.as_bytes());
        Some(Self(bytes))
    }
}

impl Default for ActorRef {
    fn default() -> Self {
        Self::new()
    }
}

impl TryFrom<String> for ActorRef {
    type Error = String;

    fn try_from(value: String) -> Result<Self, Self::Error> {
        Self::parse(&value)
            .ok_or_else(|| "actor reference must be a 21-character URL-safe NanoID".to_owned())
    }
}

impl From<ActorRef> for String {
    fn from(value: ActorRef) -> Self {
        value.to_string()
    }
}

impl fmt::Debug for ActorRef {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(formatter, "@{}", self.as_str())
    }
}

impl fmt::Display for ActorRef {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        formatter.write_str(self.as_str())
    }
}

/// Owned actor data, never a shared Rune heap object or execution handle.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(
    tag = "type",
    content = "data",
    rename_all = "snake_case",
    deny_unknown_fields
)]
pub enum Value {
    Null,
    Bool(bool),
    Integer(i64),
    String(String),
    List(Vec<Value>),
    Record(BTreeMap<String, Value>),
    ActorRef(ActorRef),
}

impl Value {
    pub fn validate(&self) -> Result<(), String> {
        fn visit(
            value: &Value,
            depth: usize,
            nodes: &mut usize,
            bytes: &mut usize,
        ) -> Result<(), String> {
            *nodes += 1;
            if depth > MAX_VALUE_DEPTH || *nodes > MAX_VALUE_NODES {
                return Err("actor value exceeds depth or node limit".to_owned());
            }
            match value {
                Value::String(value) => *bytes += value.len(),
                Value::List(values) => {
                    for value in values {
                        visit(value, depth + 1, nodes, bytes)?;
                    }
                }
                Value::Record(values) => {
                    for (key, value) in values {
                        *bytes += key.len();
                        visit(value, depth + 1, nodes, bytes)?;
                    }
                }
                _ => {}
            }
            if *bytes > MAX_VALUE_BYTES {
                return Err("actor value exceeds string byte limit".to_owned());
            }
            Ok(())
        }
        visit(self, 0, &mut 0, &mut 0)
    }
}

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

    #[test]
    fn generated_references_round_trip_and_are_distinct_values() {
        let reference = ActorRef::new();
        assert_eq!(ActorRef::parse(reference.as_str()), Some(reference));
        assert_eq!(format!("{reference:?}"), format!("@{reference}"));
        assert_ne!(
            Value::ActorRef(reference),
            Value::String(reference.to_string())
        );
        assert!(ActorRef::parse("too-short").is_none());
        assert!(ActorRef::parse("abcdefghijklmnopqrst!").is_none());
    }

    #[test]
    fn value_limits_are_inclusive() {
        let mut value = Value::Null;
        for _ in 0..MAX_VALUE_DEPTH {
            value = Value::List(vec![value]);
        }
        assert!(value.validate().is_ok());
        assert!(Value::List(vec![value]).validate().is_err());
        assert!(
            Value::List(vec![Value::Null; MAX_VALUE_NODES - 1])
                .validate()
                .is_ok()
        );
        assert!(
            Value::List(vec![Value::Null; MAX_VALUE_NODES])
                .validate()
                .is_err()
        );
        assert!(
            Value::String("x".repeat(MAX_VALUE_BYTES))
                .validate()
                .is_ok()
        );
        assert!(
            Value::String("x".repeat(MAX_VALUE_BYTES + 1))
                .validate()
                .is_err()
        );
    }
}