zigzag-engine/examples/trace_merge.rs

//! Trace the merge topology of half_braid
//!
//! Run with: cargo run --example trace_merge

use std::fs;
use zigzag_engine::diagram::Diagram;
use zigzag_engine::import::load_homotopy_diagram_n;

fn main() {
    let json = fs::read_to_string("fixtures/half_braid.json")
        .expect("Failed to read half_braid.json");
    let half_braid = load_homotopy_diagram_n(&json)
        .expect("Failed to parse");

    println!("=== MERGE TOPOLOGY ANALYSIS ===\n");

    // Source 2-diagram structure
    if let Diagram::DiagramN(src) = half_braid.source() {
        println!("SOURCE 2-diagram ({} cospans):", src.cospans.len());
        println!("Heights: r0, s0, r1, s1, r2");
        println!();

        // Print y-coordinates for each height
        println!("Height mappings (using layout_coords logic):");
        for i in 0..=src.cospans.len() {
            let y = (i as f64) - 1.0;
            println!("  r{}: y = {:.1}", i, y);
            if i < src.cospans.len() {
                let y_sing = i as f64;
                println!("  s{}: y = {:.1} ← SCALAR HERE", i, y_sing);
            }
        }
        println!();
    }

    // Target 2-diagram structure
    let target = half_braid.target();
    if let Diagram::DiagramN(tgt) = &target {
        println!("TARGET 2-diagram ({} cospans):", tgt.cospans.len());
        println!("Heights: r0, s0, r1");
        println!();

        println!("Height mappings:");
        for i in 0..=tgt.cospans.len() {
            let y = (i as f64) - 1.0;
            println!("  r{}: y = {:.1}", i, y);
            if i < tgt.cospans.len() {
                let y_sing = i as f64;
                println!("  s{}: y = {:.1} ← MERGED SCALAR HERE", i, y_sing);
            }
        }
        println!();
    }

    println!("=== VISIBLE ELEMENT ANALYSIS ===\n");

    println!("The 2 VERTICES (geom_dim=0) are the TWO INPUT SCALARS:");
    println!("  vertex (s0,s0,s0): z=-0.5, the FIRST scalar from source s0");
    println!("  vertex (s1,s0,s0): z=+0.5, the SECOND scalar from source s1");
    println!();

    println!("The 3 WIRES (geom_dim=1) are the BOUNDARIES between regions:");
    println!("  wire (r0,s0,s0): z=-1.0, LEFT boundary (below both scalars)");
    println!("  wire (r1,s0,s0): z= 0.0, MIDDLE boundary (between the two scalars)");
    println!("  wire (r2,s0,s0): z=+1.0, RIGHT boundary (above both scalars)");
    println!();

    println!("=== Y-SHAPE TOPOLOGY ===\n");

    println!("The MERGE contracts source heights r0,s0,r1,s1,r2 into target heights r0,s0,r1");
    println!();
    println!("Mapping:");
    println!("  Source r0 (y=-1) → Target r0 (y=-1)  [PRESERVED]");
    println!("  Source s0 (y= 0) → Target s0 (y= 0)  [MERGED INTO]");
    println!("  Source r1 (y= 0) → Target s0 (y= 0)  [ABSORBED]");
    println!("  Source s1 (y= 1) → Target s0 (y= 0)  [MERGED INTO]");
    println!("  Source r2 (y=+1) → Target r1 (y= 0)  [CONTRACTED DOWN]");
    println!();

    println!("For the 3 visible wires:");
    println!();
    println!("Wire r0 (z=-1, LEFT EDGE):");
    println!("  Source endpoint (x=-1): y=-1 (at source height r0)");
    println!("  Merge waypoint  (x= 0): y= 0 (at merge height s0)");
    println!("  Target endpoint (x=+1): y=-1 (at target height r0)");
    println!("  → This wire DIPS DOWN to the merge then back up");
    println!();

    println!("Wire r1 (z=0, MIDDLE/STEM):");
    println!("  Source endpoint (x=-1): y= 0 (at source height r1, between s0 and s1)");
    println!("  Merge waypoint  (x= 0): y= 0 (at merge height s0)");
    println!("  Target endpoint (x=+1): y= 0 (at target height s0)");
    println!("  → This is the STEM - stays at y=0 throughout");
    println!();

    println!("Wire r2 (z=+1, RIGHT EDGE):");
    println!("  Source endpoint (x=-1): y=+1 (at source height r2)");
    println!("  Merge waypoint  (x= 0): y= 0 (at merge height s0)");
    println!("  Target endpoint (x=+1): y= 0 (at target height r1)");
    println!("  → This wire comes DOWN from above into the merge");
    println!();

    println!("=== THE Y-SHAPE ===\n");
    println!("Looking at y-z plane (height vs depth) at different x (time) slices:\n");

    println!("At SOURCE (x=-1):           At MERGE (x=0):           At TARGET (x=+1):");
    println!("                                                                        ");
    println!("y=+1  ──●r2──               y=+1                      y=+1              ");
    println!("        │                         ╲                                     ");
    println!("        │                          ╲                                    ");
    println!("y= 0  ──●r1──  ←s1 scalar   y= 0   ●●●  (merge)       y= 0  ──●r1,r2──  ");
    println!("        │                          ╱     ↑                    ↑        ");
    println!("        │                         ╱   vertices              merged     ");
    println!("y=-1  ──●r0──  ←s0 scalar   y=-1                      y=-1  ──●r0──     ");
    println!("                                                                        ");
    println!("     z: -1  0  +1                -1  0  +1                 -1  0  +1    ");
}