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//! # Micro-pass: merge single-origin gotos into their parent to reduce CFG graph size.
use itertools::Itertools;

use crate::ids::Vector;
use crate::transform::TransformCtx;
use crate::ullbc_ast::*;

use super::ctx::UllbcPass;

pub struct Transform;
impl UllbcPass for Transform {
    fn transform_body(&self, ctx: &mut TransformCtx<'_>, body: &mut ExprBody) {
        // Check the option which instructs to ignore this pass
        if ctx.options.no_merge_goto_chains {
            return;
        }

        // Compute for each block the number of blocks that points to it.
        let mut antecedents: Vector<BlockId, usize> = body.body.map_ref(|_| 0);
        for block in body.body.iter() {
            for target in block.targets() {
                *antecedents.get_mut(target).unwrap() += 1;
            }
        }
        // Merge blocks with a single antecedent into their antecedent.
        let mut to_process = body.body.iter_indices().collect_vec();
        while let Some(id) = to_process.pop() {
            let Some(block) = body.body.get(id) else {
                continue;
            };
            let RawTerminator::Goto { target } = block.terminator.content else {
                continue;
            };
            if antecedents[target] != 1 {
                continue;
            };
            let target = body.body.remove(target).unwrap();
            let source = &mut body.body[id];
            source.statements.extend(target.statements);
            source.terminator = target.terminator;
            // We updated the terminator so we may need to process this again.
            to_process.push(id);
        }
    }
}