OpenNest/docs/plans/2026-03-08-remnant-fill-investigation.md

Remnant Fill Optimization — Investigation & Fix

Status: Both fixes done

Problem 1 (FIXED): N0308-008 hinge plate remnant

NestEngine.Fill(NestItem, Box) got 7 parts in a 4.7x35.0 remnant strip where manual nesting gets 8 using a staggered brick pattern with alternating rotations.

Test Case

load_nest("C:/Users/AJ/Desktop/N0308-008.zip")
fill_remnants(0, "Converto 3 YRD DUMPERSTER HINGE PLATE #2")  → was 7, now 9

Reference: C:/Users/AJ/Desktop/N0308-008 - Copy.zip (83 parts total, 8 in remnant).

Root Cause Found

• FillLinear rotation sweep works correctly — tested at 1° resolution, max is always 7
• The reference uses a staggered pair pattern (alternating 90°/270° rotations with horizontal offset)
• FillWithPairs generates ~2572 pair candidates but only tried top 50 sorted by minimum bounding area
• The winning pair ranked ~882nd — excluded by the Take(50) cutoff
• Top-50-by-area favors compact pairs for full-plate tiling, not narrow pairs suited for remnant strips

Fix Applied (in OpenNest.Engine/NestEngine.cs)

Added SelectPairCandidates() method:

1. Always includes standard top 50 pairs by area (no change for full-plate fills)
2. When work area is narrow (shortSide < plateShortSide * 0.5), includes all pairs whose shortest side fits the strip width
3. Updated FillWithPairs() to call SelectPairCandidates() instead of Take(50)

Results

• Remnant fill: 7 → 9 parts (beats reference of 8, with partial pattern fill)
• Full-plate fill: 75 parts (unchanged, no regression)
• Remnant fill time: ~440ms
• Overlap check: PASS

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Problem 2 (FIXED): N0308-017 PT02 remnant

N0308-017.zip — 54 parts on a 144x72 plate. Two remnant areas:

• Remnant 0: (119.57, 0.75) 24.13x70.95 — end-of-sheet strip
• Remnant 1: (0.30, 66.15) 143.40x5.55 — bottom strip

Drawing "4980 A24 PT02" has bbox 10.562x15.406. Engine filled 8 parts (2 cols × 4 rows) in remnant 0. Reference (N0308-017 - Copy.zip) has 10 parts using alternating 0°/180° rows.

Investigation

1. Tested PT02 in remnant isolation → still 8 parts (not a multi-drawing ordering issue)
2. Brute-forced all 7224 pair candidates → max was 8 (no pair yields >8 with full-pattern-only tiling)
3. Tried finer offset resolution (0.05" step) across 0°/90°/180°/270° → still max 8
4. Analyzed reference nest (N0308-017 - Copy.zip): 64 PT02 parts on full plate, 10 in remnant area

Root Cause Found

The reference uses a 0°/170° staggered pair pattern that tiles in 5 rows × 2 columns:

• Rows at y: 0.75, 14.88, 28.40, 42.53, 56.06 (alternating 0° and 170°)
• Pattern copy distance: ~27.65" (pair tiling distance)
• 2 full pairs = 8 parts, top at ~58.56"
• Remaining height: 71.70 - 58.56 = ~13.14" — enough for 1 more row of 0° parts (height 15.41)
• But FillLinear.TilePattern only placed complete pattern copies, so the partial 3rd pair (just the 0° row) was never attempted

The pair candidate DID exist in the candidate set and was being tried. The issue was entirely in FillLinear.TilePattern — it tiled 2 complete pairs (8 parts) and stopped, even though 2 more individual parts from the next incomplete pair would still fit within the work area.

Fix Applied (in OpenNest.Engine/FillLinear.cs)

Added partial pattern fill to TilePattern():

• After tiling complete pattern copies, if the pattern has multiple parts, clone the next would-be copy
• Check each individual part's bounding box against the work area
• Add any that fit — guaranteed no overlaps by the copy distance computation

This is safe because:

• The copy distance ensures no overlaps between adjacent full copies → partial (subset) is also safe
• Parts within the same pattern copy don't overlap by construction
• Individual bounds checking catches parts that exceed the work area

Results

• PT02 remnant fill: 8 → 10 parts (matches reference)
• Hinge remnant fill: 8 → 9 parts (bonus improvement from same fix)
• Full-plate fill: 75 parts (unchanged, no regression)
• All overlap checks: PASS
• PT02 fill time: ~32s (unchanged, dominated by pair candidate evaluation)

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Files Modified

• OpenNest.Engine/NestEngine.cs — Added SelectPairCandidates(), updated FillWithPairs(), rotation sweep (pre-existing change)
• OpenNest.Engine/FillLinear.cs — Added partial pattern fill to TilePattern()

Temp Files to Clean Up

• OpenNest.Test/ — temporary test console project (can be deleted or kept for debugging)