polydat-core 0.3.2

// Copyright 2024-2026 Jonathan Shook
// SPDX-License-Identifier: Apache-2.0

//! Operator IR — spec §9.1.
//!
//! Every well-formed comprehension AST compiles to a finite
//! sequence of these 8 opcodes. Every operator is a stream
//! transducer; operands flow as tuple streams via
//! `advance() -> Option<Tuple>`, never as materialized
//! `Vec<Tuple>`. The two materialization barriers — non-Lex
//! `ORDER_MATERIALIZE` and `ZIP(Cycle)`'s shorter-child
//! buffering — are called out explicitly.

use serde::{Deserialize, Serialize};

use crate::iteration::comprehension::source::Source;
use crate::iteration::comprehension::strategy::{StrategyName, ZipMode};

/// The 8-opcode IR set (spec §9.1).
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
#[serde(tag = "op", rename_all = "snake_case")]
pub enum Op {
    /// Push a single-name tuple stream produced by `source`.
    /// Streaming; O(1) per pull above the source's own state.
    PushClause {
        /// The name the stream binds.
        name: String,
        /// Where its values come from.
        source: Source,
    },

    /// Replace the top-N stream operands with one stream that
    /// enumerates their cross product in Lex order. Streaming.
    Cartesian {
        /// Operands combined.
        n: usize,
    },

    /// Replace the top-N stream operands with their lockstep
    /// diagonal. Streaming under Strict/Truncate; `Cycle`
    /// buffers each non-longest child.
    Zip {
        /// Operands combined.
        n: usize,
        /// The length policy.
        mode: ZipMode,
    },

    /// Replace the top-N stream operands with a stream that
    /// concatenates them in operand order. Streaming.
    Union {
        /// Operands concatenated.
        n: usize,
    },

    /// Wrap the top operand with a per-tuple predicate check.
    /// Streaming.
    Filter {
        /// The predicate, a boolean expression over the tuple.
        predicate: String,
    },

    /// Wrap the top operand with a counter / pass-through.
    /// Used for `order(Lex, _)` per spec §10.2 R1; this is
    /// the "streaming" order opcode.
    OrderStreaming {
        /// The streaming order's kind.
        kind: OrderStreamingKind,
        /// The output cap, if any.
        truncation: Option<u64>,
    },

    /// MATERIALIZATION BARRIER. Build a working set sufficient
    /// for the strategy, apply the strategy, emit permuted
    /// tuples. Truncation is the output cap.
    ///
    /// Per spec §10.2 R2: when the input is index-addressable
    /// and the strategy has a closed-form push-down rule, the
    /// working set shrinks from O(input) to O(output) — the
    /// interpreter realizes this by drawing strategy-specific
    /// multi-indices and looking each up against the input's
    /// `IndexFn` rather than materializing the full input.
    /// Whether R2 fires is encoded in `indexed`.
    ///
    /// `input_index_fn` carries the upstream comprehension's
    /// addressing scheme (per spec §10.7.6 / §10.7.8) so the
    /// strategy's indexed-form algorithms can dispatch
    /// correctly without re-deriving the shape from observed
    /// tuples (which would lose multi-axis lattice structure
    /// after the flat materialization). `None` when the
    /// upstream metadata propagator couldn't claim a closed-
    /// form addressing function.
    OrderMaterialize {
        /// The strategy applied.
        strategy: StrategyName,
        /// The output cap, if any.
        truncation: Option<u64>,
        /// `true` when R2 push-down applies: the interpreter
        /// should use the strategy's indexed form (draw
        /// multi-indices, look up via input IndexFn).
        /// `false` for the naïve form (materialize input,
        /// then apply).
        indexed: bool,
        /// Upstream input's IndexFn at compile time (spec
        /// §10.7.6). The interpreter passes this into the
        /// [`crate::iteration::comprehension::strategies::EvaluatedInput`]
        /// it builds for [`crate::iteration::comprehension::strategies::Strategy::apply`].
        input_index_fn: Option<crate::iteration::comprehension::metadata::IndexFn>,
    },

    /// Bind the top stream as the comprehension's result.
    /// Must be the last opcode in a well-formed Program.
    Dispense,
}

/// Variant marker for [`Op::OrderStreaming`]. Today only Lex
/// is streaming (per spec §6.2's "streaming order" table);
/// the enum exists so future streaming strategies can land
/// without changing the IR opcode set.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum OrderStreamingKind {
    /// Lexicographic order: the natural enumeration, counted.
    Lex,
}

impl Op {
    /// Arity for stack-effect computation: how many stream
    /// operands this opcode pops, and how many it pushes.
    /// Always pushes 1 for stream-producing ops; `Dispense`
    /// pushes 0 (it consumes the final stream).
    pub fn stack_effect(&self) -> (usize, usize) {
        match self {
            Op::PushClause { .. } => (0, 1),
            Op::Cartesian { n } => (*n, 1),
            Op::Zip { n, .. } => (*n, 1),
            Op::Union { n } => (*n, 1),
            Op::Filter { .. } => (1, 1),
            Op::OrderStreaming { .. } => (1, 1),
            Op::OrderMaterialize { .. } => (1, 1),
            Op::Dispense => (1, 0),
        }
    }

    /// `true` if this opcode is a materialization barrier per
    /// spec §6.2 + §6.3. Used by the bounds checker.
    pub fn is_barrier(&self) -> bool {
        matches!(
            self,
            Op::OrderMaterialize { .. }
                | Op::Zip {
                    mode: ZipMode::Cycle,
                    ..
                }
        )
    }
}

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

    #[test]
    fn stack_effect_basics() {
        assert_eq!(
            Op::PushClause {
                name: "k".into(),
                source: Source::Literal { values: vec![] },
            }
            .stack_effect(),
            (0, 1)
        );
        assert_eq!(Op::Cartesian { n: 3 }.stack_effect(), (3, 1));
        assert_eq!(Op::Dispense.stack_effect(), (1, 0));
    }

    #[test]
    fn barrier_classification() {
        assert!(
            Op::OrderMaterialize {
                strategy: StrategyName::Halton,
                truncation: Some(10),
                indexed: true,
                input_index_fn: None,
            }
            .is_barrier()
        );
        assert!(
            Op::Zip {
                n: 2,
                mode: ZipMode::Cycle
            }
            .is_barrier()
        );
        assert!(
            !Op::Zip {
                n: 2,
                mode: ZipMode::Strict
            }
            .is_barrier()
        );
        assert!(
            !Op::OrderStreaming {
                kind: OrderStreamingKind::Lex,
                truncation: None,
            }
            .is_barrier()
        );
    }

    #[test]
    fn serde_round_trip() {
        let op = Op::OrderMaterialize {
            strategy: StrategyName::Halton,
            truncation: Some(50),
            indexed: true,
            input_index_fn: Some(
                crate::iteration::comprehension::metadata::IndexFn::Lattice {
                    axis_sizes: vec![10, 5],
                },
            ),
        };
        let json = serde_json::to_string(&op).unwrap();
        let back: Op = serde_json::from_str(&json).unwrap();
        assert_eq!(op, back);
    }
}