//! Incomplete but useful parser and generator for Postgres flavored SQL
//! expressions and more, based on a modified version of the
//! [official Pest SQL grammar](https://github.com/pest-parser/pest/blob/79dd30d11aab6f0fba3cd79bd48f456209b966b3/grammars/src/grammars/sql.pest).
//!
//! ## Example
//!
//! ```
//! use phono_pestgros::{ArithOp, BoolOp, Datum, Expr, InfixOp};
//!
//! # fn main() -> Result<(), Box<dyn Error>> {
//! let expr = Expr::try_from("3 + 5 < 10")?;
//!
//! assert_eq!(expr, Expr::Infix {
//!   lhs: Box::new(Expr::Infix {
//!     lhs: Box::new(Expr::Literal(Datum::Numeric(Some(3.into())))),
//!     op: InfixOp::ArithInfix(ArithOp::Add),
//!     rhs: Box::new(Expr::Literal(Datum::Numeric(Some(5.into())))),
//!   }),
//!   op: InfixOp::BoolInfix(BoolOp::Lt),
//!   rhs: Box::new(Expr::Literal(Datum::Numeric(Some(10.into())))),
//! });
//!
//! assert_eq!(QueryBuilder::try_from(expr).sql(), "(($1) + ($2)) < ($3)");
//! # Ok(())
//! # }
//! ```

use std::{str::FromStr, sync::LazyLock};

use bigdecimal::BigDecimal;
use pest::{
    Parser as _,
    iterators::{Pair, Pairs},
    pratt_parser::PrattParser,
};
use pest_derive::Parser;

pub use crate::datum::Datum;

mod datum;
mod query_builders;

#[cfg(test)]
mod fragment_tests;
#[cfg(test)]
mod func_invocation_tests;
#[cfg(test)]
mod identifier_tests;
#[cfg(test)]
mod literal_tests;
#[cfg(test)]
mod op_tests;

/// Given a raw identifier (such as a table name, column name, etc.), format it
/// so that it may be safely interpolated into a SQL query.
///
/// Note that in PostgreSQL, unquoted identifiers are case-insensitive (or,
/// rather, they are always implicitly converted to lowercase), while quoted
/// identifiers are case-sensitive. The caller of this function is responsible
/// for performing conversion to lowercase as appropriate.
pub fn escape_identifier(identifier: &str) -> String {
    // Escaping identifiers for Postgres is fairly easy, provided that the input is
    // already known to contain no invalid multi-byte sequences. Backslashes may
    // remain as-is, and embedded double quotes are escaped simply by doubling
    // them (`"` becomes `""`). Refer to the PQescapeInternal() function in
    // libpq (fe-exec.c) and Diesel's PgQueryBuilder::push_identifier().
    format!("\"{0}\"", identifier.replace('"', "\"\""))
}

/// Decodes a SQL representation of an identifier. If the input is unquoted, it
/// is converted to lowercase. If it is double quoted, the surrounding quotes
/// are stripped and escaped inner double quotes (double-double quotes, if you
/// will) are converted to single-double quotes. The opposite of
/// [`escape_identifier`], sort of.
///
/// Assumes that the provided identifier is well-formed. Basic gut checks are
/// performed, but they are non-exhaustive.
///
/// `U&"..."`-style escaped Unicode identifiers are not yet supported.
fn parse_ident(value: &str) -> String {
    assert!(
        !value.to_lowercase().starts_with("u&"),
        "escaped Unicode identifiers are not supported"
    );
    if value.starts_with('"') {
        assert!(value.ends_with('"'), "malformed double-quoted identifier");
        {
            // Strip first and last characters.
            let mut chars = value.chars();
            chars.next();
            chars.next_back();
            chars.as_str()
        }
        .replace(r#""""#, r#"""#)
    } else {
        // TODO: assert validity with regex
        value.to_lowercase()
    }
}

/// Decodes a single-quoted string literal. Removes surrounding quotes and
/// replaces embedded single quotes (double-single quotes) with single-single
/// quotes.
///
/// Assumes that the provided identifier is well-formed. Basic gut checks are
/// performed, but they are non-exhaustive.
///
/// `E'...'`-style, dollar-quoted, and other (relatively) uncommon formats for
/// text literals are not yet supported.
fn parse_text_literal(value: &str) -> String {
    assert!(value.starts_with('\'') && value.ends_with('\''));
    {
        // Strip first and last characters.
        let mut chars = value.chars();
        chars.next();
        chars.next_back();
        chars.as_str()
    }
    .replace("''", "'")
}

/// Primary parser and code generation for [`Rule`] types.
#[derive(Parser)]
#[grammar = "src/grammar.pest"]
struct PsqlParser;

/// Secondary parser configuration for handling operator precedence.
static PRATT_PARSER: LazyLock<PrattParser<Rule>> = LazyLock::new(|| {
    use pest::pratt_parser::{
        Assoc::{Left, Right},
        Op,
    };

    PrattParser::new()
        .op(Op::infix(Rule::Or, Left))
        .op(Op::infix(Rule::Between, Left))
        .op(Op::infix(Rule::And, Left))
        .op(Op::prefix(Rule::UnaryNot))
        .op(Op::infix(Rule::Eq, Right)
            | Op::infix(Rule::NotEq, Right)
            | Op::infix(Rule::Gt, Right)
            | Op::infix(Rule::GtEq, Right)
            | Op::infix(Rule::Lt, Right)
            | Op::infix(Rule::LtEq, Right)
            | Op::infix(Rule::In, Right))
        // Official Pest example overstates the concat operator's precedence. It
        // should be lower precedence than add/subtract.
        .op(Op::infix(Rule::ConcatInfixOp, Left))
        .op(Op::infix(Rule::Add, Left) | Op::infix(Rule::Subtract, Left))
        .op(Op::infix(Rule::Multiply, Left) | Op::infix(Rule::Divide, Left))
        .op(Op::postfix(Rule::IsNullPostfix))
});

/// Represents a SQL expression. An expression is a collection of values and
/// operators that theoretically evaluates to some value, such as a boolean
/// condition, an object name, or a string dynamically derived from other
/// values. An expression is *not* a complete SQL statement, command, or query.
#[derive(Clone, Debug, PartialEq)]
pub enum Expr {
    Infix {
        lhs: Box<Expr>,
        op: InfixOp,
        rhs: Box<Expr>,
    },
    Literal(Datum),
    ObjName(Vec<String>),
    FnCall {
        name: Vec<String>,
        args: FnArgs,
    },
    Not(Box<Expr>),
    Nullness {
        is_null: bool,
        expr: Box<Expr>,
    },
}

impl TryFrom<&str> for Expr {
    type Error = ParseError;

    fn try_from(value: &str) -> Result<Self, Self::Error> {
        // `ExprRoot` is a silent rule which simply dictates that the inner
        // `Expr` rule must consume the entire input.
        let pairs = PsqlParser::parse(Rule::ExprRoot, value)?;
        parse_expr_pairs(pairs)
    }
}

#[derive(Clone, Copy, Debug, PartialEq)]
pub enum InfixOp {
    ArithInfix(ArithOp),
    BoolInfix(BoolOp),
}

#[derive(Clone, Copy, Debug, PartialEq)]
pub enum ArithOp {
    Add,
    Concat,
    Div,
    Mult,
    Sub,
}

#[derive(Clone, Copy, Debug, PartialEq)]
pub enum BoolOp {
    And,
    Or,
    Eq,
    Gt,
    Gte,
    Lt,
    Lte,
    Neq,
}

#[derive(Clone, Debug, PartialEq)]
pub enum FnArgs {
    CountAsterisk,
    Exprs {
        /// `true` for aggregator invocations with the `DISTINCT` keyword
        /// specified.
        distinct_flag: bool,
        exprs: Vec<Expr>,
    },
}

/// Recursive helper, which does most of the work to convert [`pest`]'s pattern
/// matching output to a usable syntax tree.
fn parse_expr_pairs(expr_pairs: Pairs<'_, Rule>) -> Result<Expr, ParseError> {
    PRATT_PARSER
        .map_primary(|pair| match pair.as_rule() {
            Rule::Expr | Rule::ExpressionInParentheses => parse_expr_pairs(pair.into_inner()),
            Rule::Decimal | Rule::Double | Rule::Integer | Rule::Unsigned => Ok(Expr::Literal(
                Datum::Numeric(Some(BigDecimal::from_str(pair.as_str()).expect(
                    "parsed numeric values should always be convertible to BigDecimal",
                ))),
            )),
            Rule::SingleQuotedString => Ok(Expr::Literal(Datum::Text(Some(parse_text_literal(pair.as_str()))))),
            Rule::IdentifierWithOptionalContinuation => {
                let mut name: Vec<String> = vec![];
                let mut fn_args: Option<FnArgs> = None;
                let inner = pair.into_inner();
                for inner_pair in inner {
                    match inner_pair.as_rule() {
                        Rule::Identifier => {
                            name.push(parse_ident(inner_pair.as_str()));
                        }
                        Rule::QualifiedIdentifierContinuation => {
                            let ident_cont = inner_pair.as_str();
                            assert!(
                                ident_cont.starts_with('.'),
                                "QualifiedIdentifierContinuation should always start with the infix dot",
                            );
                            name.push(parse_ident({
                                // Strip leading dot.
                                let mut chars = ident_cont.chars();
                                chars.next();
                                chars.as_str()
                            }));
                        }
                        Rule::FunctionInvocationContinuation => {
                            fn_args = Some(parse_function_invocation_continuation(inner_pair)?);
                        }
                        _ => unreachable!(
                            "IdentifierWithOptionalContinuation has only 3 valid child rules",
                        ),
                    }
                }
                Ok(if let Some(fn_args) = fn_args {
                    Expr::FnCall { name, args: fn_args }
                } else {
                    Expr::ObjName(name)
                })
            }
            rule => Err(ParseError::UnknownRule(rule)),
        })
        .map_infix(|lhs, op, rhs| Ok(Expr::Infix {
            lhs: Box::new(lhs?),
            op: match op.as_rule() {
                Rule::Add => InfixOp::ArithInfix(ArithOp::Add),
                Rule::ConcatInfixOp => InfixOp::ArithInfix(ArithOp::Concat),
                Rule::Divide => InfixOp::ArithInfix(ArithOp::Div),
                Rule::Multiply => InfixOp::ArithInfix(ArithOp::Mult),
                Rule::Subtract => InfixOp::ArithInfix(ArithOp::Sub),
                Rule::And => InfixOp::BoolInfix(BoolOp::And),
                Rule::Eq => InfixOp::BoolInfix(BoolOp::Eq),
                Rule::Gt => InfixOp::BoolInfix(BoolOp::Gt),
                Rule::GtEq => InfixOp::BoolInfix(BoolOp::Gte),
                Rule::Lt => InfixOp::BoolInfix(BoolOp::Lt),
                Rule::LtEq => InfixOp::BoolInfix(BoolOp::Lte),
                Rule::NotEq => InfixOp::BoolInfix(BoolOp::Neq),
                Rule::Or => InfixOp::BoolInfix(BoolOp::Or),
                rule => Err(ParseError::UnknownRule(rule))?,
            },
            rhs: Box::new(rhs?),
        }))
        .map_prefix(|op, child| Ok(match op.as_rule() {
            Rule::UnaryNot => Expr::Not(Box::new(child?)),
            rule => Err(ParseError::UnknownRule(rule))?,
        }))
        .map_postfix(|child, op| Ok(match op.as_rule() {
            Rule::IsNullPostfix => Expr::Nullness {
                is_null: op
                    .into_inner()
                    .next()
                    .map(|inner| inner.as_rule()) != Some(Rule::NotFlag),
                expr: Box::new(child?),
            },
            rule => Err(ParseError::UnknownRule(rule))?,
        }))
        .parse(expr_pairs)
}

fn parse_function_invocation_continuation(pair: Pair<'_, Rule>) -> Result<FnArgs, ParseError> {
    let mut cont_inner_iter = pair.into_inner();
    let fn_args = if let Some(cont_inner) = cont_inner_iter.next() {
        match cont_inner.as_rule() {
            Rule::FunctionArgs => {
                let mut distinct_flag = false;
                let mut exprs: Vec<Expr> = vec![];
                for arg_inner in cont_inner.into_inner() {
                    match arg_inner.as_rule() {
                        Rule::Distinct => {
                            distinct_flag = true;
                        }
                        Rule::Expr => {
                            exprs.push(parse_expr_pairs(arg_inner.into_inner())?);
                        }
                        _ => unreachable!(
                            "only valid children of FunctionArgs are Distinct and Expr"
                        ),
                    }
                }
                FnArgs::Exprs {
                    distinct_flag,
                    exprs,
                }
            }
            Rule::CountAsterisk => FnArgs::CountAsterisk,
            _ => unreachable!(
                "only valid children of FunctionInvocationContinuation are FunctionArgs and CountAsterisk"
            ),
        }
    } else {
        FnArgs::Exprs {
            distinct_flag: false,
            exprs: vec![],
        }
    };
    assert!(
        cont_inner_iter.next().is_none(),
        "function should have consumed entire FunctionInvocationContinuation pair",
    );
    Ok(fn_args)
}

#[derive(Clone, Debug, PartialEq, thiserror::Error)]
#[error("parse error")]
pub enum ParseError {
    #[error("unknown rule")]
    UnknownRule(Rule),
    #[error("pest failed to parse: {0}")]
    Pest(pest::error::Error<Rule>),
}

impl From<pest::error::Error<Rule>> for ParseError {
    fn from(value: pest::error::Error<Rule>) -> Self {
        Self::Pest(value)
    }
}