"""Pure-logic tests for the mid-print tray-split math (#1793).

The helper lives in ``backend/app/utils/tray_split.py`` and is exercised
by both inventory backends (``usage_tracker`` and ``spoolman_tracking``).
These tests pin the algorithm so a change in one caller can't silently
break the other — cross-inventory parity is a HARD RULE for this project.
"""

from __future__ import annotations

from backend.app.utils.tray_split import compute_tray_split_grams


class TestComputeTraySplitGrams:
    """Segment-attribution algorithm — gcode preferred, linear fallback, equal split."""

    def test_empty_tray_changes_returns_empty(self):
        assert (
            compute_tray_split_grams(
                tray_changes=[],
                total_weight=100.0,
                slot_id=1,
                layer_usage=None,
                density=1.24,
                diameter=1.75,
                total_layers=200,
                last_layer_num=200,
            )
            == []
        )

    def test_single_segment_charges_everything_to_that_tray(self):
        segments = compute_tray_split_grams(
            tray_changes=[(0, 0)],
            total_weight=72.56,
            slot_id=1,
            layer_usage=None,
            density=1.24,
            diameter=1.75,
            total_layers=100,
            last_layer_num=100,
        )
        assert segments == [(0, 0, 72.56)]

    def test_two_segments_linear_split_by_layer_ratio(self):
        # Runout at layer 37 of 100 total; no gcode available → linear.
        # Segment 0 (tray 0, layers 0-37) = 100 * 37/100 = 37g
        # Segment 1 (tray 1, layers 37-end) = 100 - 37 = 63g (remainder)
        segments = compute_tray_split_grams(
            tray_changes=[(0, 0), (1, 37)],
            total_weight=100.0,
            slot_id=1,
            layer_usage=None,
            density=1.24,
            diameter=1.75,
            total_layers=100,
            last_layer_num=100,
        )
        assert segments == [(0, 0, 37.0), (1, 1, 63.0)]

    def test_two_segments_gcode_preferred_over_linear(self):
        # layer_usage stores mm of filament extruded per (layer, filament_id).
        # Values are cumulative-per-key inside get_cumulative_usage_at_layer.
        # 20 layers, filament_id=0 (slot_id=1 → filament_id 0):
        #   layer 10 → 100mm cumulative
        #   layer 20 → 300mm cumulative
        # tray change at layer 10 → seg 0 spans layers 0-10 (mm 0 → 100),
        #                            seg 1 spans layers 10-end.
        # mm_to_grams(100, 1.75, 1.24) ≈ 0.298g; last segment absorbs the rest.
        layer_usage = {
            5: {0: 50.0},
            10: {0: 100.0},
            15: {0: 200.0},
            20: {0: 300.0},
        }
        segments = compute_tray_split_grams(
            tray_changes=[(0, 0), (1, 10)],
            total_weight=1.0,  # sentinel — we assert the seg1 remainder
            slot_id=1,
            layer_usage=layer_usage,
            density=1.24,
            diameter=1.75,
            total_layers=20,
            last_layer_num=20,
        )
        # Seg 0 charged from gcode delta (mm 0 → 100).
        # Seg 1 gets total_weight - seg0 as remainder.
        assert segments[0][0] == 0
        assert segments[0][1] == 0  # tray 0
        assert segments[0][2] > 0  # non-zero gcode contribution
        assert segments[1][0] == 1
        assert segments[1][1] == 1  # tray 1
        # Sum equals the input total by construction (last segment absorbs).
        assert round(segments[0][2] + segments[1][2], 6) == 1.0

    def test_three_segments_last_absorbs_rounding_drift(self):
        # 100g over three segments at layers 30 and 60 of 90; linear fallback.
        # Seg 0: 100 * 30/90 = 33.3333...
        # Seg 1: 100 * 30/90 = 33.3333...
        # Seg 2: remainder = 100 - 66.6666... = 33.3333... — exact by construction
        segments = compute_tray_split_grams(
            tray_changes=[(0, 0), (1, 30), (2, 60)],
            total_weight=100.0,
            slot_id=1,
            layer_usage=None,
            density=1.24,
            diameter=1.75,
            total_layers=90,
            last_layer_num=90,
        )
        assert len(segments) == 3
        assert round(sum(g for _, _, g in segments), 6) == 100.0
        assert segments[0][1] == 0
        assert segments[1][1] == 1
        assert segments[2][1] == 2

    def test_no_layer_info_at_all_falls_to_equal_split(self):
        # Denominator 0 → last-resort equal-split; last segment absorbs remainder.
        segments = compute_tray_split_grams(
            tray_changes=[(0, 0), (1, 50)],
            total_weight=90.0,
            slot_id=1,
            layer_usage=None,
            density=1.24,
            diameter=1.75,
            total_layers=0,
            last_layer_num=0,
        )
        # 90g / 2 = 45g each; sum still 90 by remainder mechanic.
        assert segments == [(0, 0, 45.0), (1, 1, 45.0)]

    def test_last_layer_num_used_when_total_layers_zero(self):
        # P1S firmware-reset scenario: total_layers=0 at completion, but the
        # captured last_layer_num survives. Should give the same linear split
        # as if total_layers had held its value (#1771 cascade).
        segments_captured = compute_tray_split_grams(
            tray_changes=[(0, 0), (1, 30)],
            total_weight=100.0,
            slot_id=1,
            layer_usage=None,
            density=1.24,
            diameter=1.75,
            total_layers=0,
            last_layer_num=100,
        )
        segments_normal = compute_tray_split_grams(
            tray_changes=[(0, 0), (1, 30)],
            total_weight=100.0,
            slot_id=1,
            layer_usage=None,
            density=1.24,
            diameter=1.75,
            total_layers=100,
            last_layer_num=100,
        )
        assert segments_captured == segments_normal

    def test_slot_id_maps_to_zero_based_filament_id_in_gcode(self):
        # slot_id 2 → filament_id 1 in layer_usage. If we mistakenly used
        # slot_id as-is, we'd read filament_id 2 which is absent → 0mm delta
        # → seg 0 gets 0, seg 1 (remainder) gets the whole total. Guard
        # against that regression.
        layer_usage = {
            5: {0: 0.0, 1: 40.0},
            10: {0: 0.0, 1: 80.0},
            20: {0: 0.0, 1: 160.0},
        }
        segments = compute_tray_split_grams(
            tray_changes=[(0, 0), (1, 10)],
            total_weight=1.0,
            slot_id=2,
            layer_usage=layer_usage,
            density=1.24,
            diameter=1.75,
            total_layers=20,
            last_layer_num=20,
        )
        # Seg 0 gcode delta on filament_id=1 is non-zero → not 0g.
        assert segments[0][2] > 0