hakorune/src/mir/builder/exprs_peek.rs

use super::{BasicBlockId, ValueId};
use crate::ast::{ASTNode, LiteralValue};

impl super::MirBuilder {
    // Peek expression lowering
    pub(super) fn build_peek_expression(
        &mut self,
        scrutinee: ASTNode,
        arms: Vec<(LiteralValue, ASTNode)>,
        else_expr: ASTNode,
    ) -> Result<ValueId, String> {
        // Evaluate scrutinee in the current block
        let scr_val = self.build_expression_impl(scrutinee)?;

        // Prepare merge and result
        let merge_block: BasicBlockId = self.block_gen.next();
        let result_val = self.value_gen.next();
        let mut phi_inputs: Vec<(BasicBlockId, ValueId)> = Vec::new();

        // Create dispatch block where we start comparing arms
        let dispatch_block = self.block_gen.next();
        // Jump from current block to dispatch (ensure terminator exists)
        let need_jump = {
            let cur = self.current_block;
            if let (Some(cb), Some(ref func)) = (cur, &self.current_function) {
                if let Some(bb) = func.blocks.get(&cb) { !bb.is_terminated() } else { true }
            } else { true }
        };
        if need_jump {
            self.emit_instruction(super::MirInstruction::Jump { target: dispatch_block })?;
        }
        self.start_new_block(dispatch_block)?;

        // If there are no arms, fall through to else directly
        if arms.is_empty() {
            let else_block = self.block_gen.next();
            self.emit_instruction(super::MirInstruction::Jump { target: else_block })?;
            self.start_new_block(else_block)?;
            let else_val = self.build_expression_impl(else_expr)?;
            phi_inputs.push((else_block, else_val));
            self.emit_instruction(super::MirInstruction::Jump { target: merge_block })?;
            self.start_new_block(merge_block)?;
            if self.is_no_phi_mode() {
                for (pred, val) in phi_inputs {
                    self.insert_edge_copy(pred, result_val, val)?;
                }
            } else {
                self.emit_instruction(super::MirInstruction::Phi { dst: result_val, inputs: phi_inputs })?;
            }
            return Ok(result_val);
        }

        // Else block to handle default case
        let else_block = self.block_gen.next();

        // Chain dispatch blocks for each arm
        let mut cur_dispatch = dispatch_block;
        for (i, (label, arm_expr)) in arms.iter().cloned().enumerate() {
            let then_block = self.block_gen.next();
            // Next dispatch (only for non-last arm)
            let next_dispatch = if i + 1 < arms.len() { Some(self.block_gen.next()) } else { None };
            let else_target = next_dispatch.unwrap_or(else_block);

            // In current dispatch block, compare and branch
            self.start_new_block(cur_dispatch)?;
            if let LiteralValue::String(s) = label {
                let lit_id = self.value_gen.next();
                self.emit_instruction(super::MirInstruction::Const { dst: lit_id, value: super::ConstValue::String(s) })?;
                let cond_id = self.value_gen.next();
                self.emit_instruction(super::MirInstruction::Compare { dst: cond_id, op: super::CompareOp::Eq, lhs: scr_val, rhs: lit_id })?;
                self.emit_instruction(super::MirInstruction::Branch { condition: cond_id, then_bb: then_block, else_bb: else_target })?;
            }

            // then arm
            self.start_new_block(then_block)?;
            let then_val = self.build_expression_impl(arm_expr)?;
            phi_inputs.push((then_block, then_val));
            self.emit_instruction(super::MirInstruction::Jump { target: merge_block })?;

            // Move to next dispatch or else block
            cur_dispatch = else_target;
        }

        // Lower else expression in else_block
        self.start_new_block(else_block)?;
        let else_val = self.build_expression_impl(else_expr)?;
        phi_inputs.push((else_block, else_val));
        self.emit_instruction(super::MirInstruction::Jump { target: merge_block })?;

        // Merge and yield result
        self.start_new_block(merge_block)?;
        if self.is_no_phi_mode() {
            for (pred, val) in phi_inputs {
                self.insert_edge_copy(pred, result_val, val)?;
            }
        } else {
            self.emit_instruction(super::MirInstruction::Phi { dst: result_val, inputs: phi_inputs })?;
        }
        Ok(result_val)
    }
}