isomesh 0.0.7

Engine-agnostic isosurface extraction: signed distance field in, triangles out. no_std, f32 and f64, no math library in the public API.
Documentation 
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212

//! Fixtures for the determinism harness.
//!
//! The two that matter are the signed-zero and NaN cases: they are where float
//! equality gives the wrong answer in each direction, and a harness that used
//! `==` would pass one and fail the other.

use super::*;
use crate::MeshSink;
use core::cell::Cell;

fn triangle(out: &mut MeshBuffer<f64>) {
    let a = out.vertex([0.0, 0.0, 0.0], [0.0, 1.0, 0.0]);
    let b = out.vertex([1.0, 0.0, 0.0], [0.0, 1.0, 0.0]);
    let c = out.vertex([0.0, 0.0, 1.0], [0.0, 1.0, 0.0]);
    out.triangle(a, b, c);
}

#[test]
fn a_deterministic_extractor_passes() {
    let report = check_determinism(triangle);
    assert!(report.is_deterministic(), "{report}");
    assert_eq!(report.vertices, 3);
    assert_eq!(report.triangles, 1);
    report.panic_if_divergent();
}

#[test]
fn an_empty_extractor_is_deterministic() {
    let report = check_determinism(|_: &mut MeshBuffer<f32>| {});
    assert!(report.is_deterministic());
    assert_eq!(report.vertices, 0);
    assert_eq!(report.triangles, 0);
}

/// The case float equality gets wrong in one direction: `+0.0 == -0.0` is true,
/// but the bit patterns differ. A sign flip on a zero coordinate is exactly what
/// a reordered summation produces, and a harness comparing with `==` would call
/// this deterministic.
#[test]
fn signed_zero_is_a_divergence() {
    let run = Cell::new(0u32);
    let report = check_determinism(|out: &mut MeshBuffer<f64>| {
        let n = run.get();
        run.set(n + 1);
        let z = if n == 0 { 0.0 } else { -0.0 };
        out.vertex([z, 0.0, 0.0], [0.0, 1.0, 0.0]);
    });

    // `0.0 == -0.0` is true, so a harness comparing with `==` would call this
    // deterministic. Bit comparison does not.
    assert!(!report.is_deterministic(), "{report}");
    match report.divergence {
        Some((RunPair::RepeatedCall, Divergence::Position { vertex, axis, .. })) => {
            assert_eq!((vertex, axis), (0, 0));
        }
        other => panic!("expected a position divergence, got {other:?}"),
    }
}

/// The case float equality gets wrong in the other direction: `NaN != NaN`, so
/// `==` would report a divergence where the bits are identical. Meshing a field
/// that produces a NaN is a bug, but it is not a *determinism* bug, and this
/// harness must not claim it is.
#[test]
fn identical_nan_bits_are_not_a_divergence() {
    let report = check_determinism(|out: &mut MeshBuffer<f64>| {
        out.vertex([f64::NAN, 0.0, 0.0], [0.0, 1.0, 0.0]);
    });

    // `NaN != NaN` is true, so a harness comparing with `==` would report a
    // divergence here. Bit comparison does not.
    assert!(report.is_deterministic(), "{report}");
}

#[test]
fn a_differing_position_is_located_exactly() {
    let run = Cell::new(0u32);
    let report = check_determinism(|out: &mut MeshBuffer<f64>| {
        let n = run.get();
        run.set(n + 1);
        out.vertex([0.0, 0.0, 0.0], [0.0, 1.0, 0.0]);
        out.vertex([1.0, if n == 0 { 2.0 } else { 3.0 }, 0.0], [0.0, 1.0, 0.0]);
    });

    match report.divergence {
        Some((
            RunPair::RepeatedCall,
            Divergence::Position {
                vertex,
                axis,
                first,
                second,
            },
        )) => {
            assert_eq!((vertex, axis), (1, 1));
            assert_eq!((first, second), (2.0, 3.0));
        }
        other => panic!("expected a position divergence, got {other:?}"),
    }
}

/// The bug class the ticket names: iteration order reaching vertex order. The
/// positions are the same set both times, in a different order.
#[test]
fn reordered_output_is_a_divergence() {
    let run = Cell::new(0u32);
    let report = check_determinism(|out: &mut MeshBuffer<f64>| {
        let n = run.get();
        run.set(n + 1);
        let order: [f64; 2] = if n == 0 { [1.0, 2.0] } else { [2.0, 1.0] };
        for x in order {
            out.vertex([x, 0.0, 0.0], [0.0, 1.0, 0.0]);
        }
    });
    assert!(!report.is_deterministic(), "{report}");
}

#[test]
fn a_differing_vertex_count_is_reported_as_such() {
    let run = Cell::new(0u32);
    let report = check_determinism(|out: &mut MeshBuffer<f32>| {
        let n = run.get();
        run.set(n + 1);
        for _ in 0..=n {
            out.vertex([0.0, 0.0, 0.0], [0.0, 1.0, 0.0]);
        }
    });

    match report.divergence {
        Some((RunPair::RepeatedCall, Divergence::VertexCount { first, second })) => {
            assert_eq!((first, second), (1, 2));
        }
        other => panic!("expected a vertex-count divergence, got {other:?}"),
    }
}

#[test]
fn a_differing_index_is_located_exactly() {
    let run = Cell::new(0u32);
    let report = check_determinism(|out: &mut MeshBuffer<f32>| {
        let n = run.get();
        run.set(n + 1);
        for _ in 0..3 {
            out.vertex([0.0, 0.0, 0.0], [0.0, 1.0, 0.0]);
        }
        if n == 0 {
            out.triangle(0, 1, 2);
        } else {
            out.triangle(0, 2, 1);
        }
    });

    match report.divergence {
        Some((RunPair::RepeatedCall, Divergence::Index { at, first, second })) => {
            assert_eq!((at, first, second), (1, 1, 2));
        }
        other => panic!("expected an index divergence, got {other:?}"),
    }
}

/// The third run's reason for existing: output that depends on the output
/// buffer's prior state rather than on the input.
///
/// The trigger here is deliberately blunt — reading the buffer's capacity, which
/// is zero when fresh and non-zero after a reset. A real extractor would reach
/// this through a cache or a scratch buffer it forgot to clear. What matters is
/// that such a thing is caught at all, since nothing else in the suite drives an
/// extractor the way the API intends it to be driven.
#[test]
fn output_depending_on_a_reused_buffer_is_a_divergence() {
    let report = check_determinism(|out: &mut MeshBuffer<f64>| {
        let reused = out.positions.capacity() > 0;
        out.vertex([if reused { 1.0 } else { 0.0 }, 0.0, 0.0], [0.0, 1.0, 0.0]);
    });

    assert!(!report.is_deterministic(), "{report}");
    let (pair, _) = report.divergence.expect("a divergence");
    assert_eq!(
        pair,
        RunPair::ReusedBuffer,
        "the first two runs both used fresh buffers and agreed"
    );
}

#[test]
#[should_panic(expected = "non-deterministic output between")]
fn panic_if_divergent_names_the_run_pair() {
    let run = Cell::new(0u32);
    check_determinism(|out: &mut MeshBuffer<f32>| {
        let n = run.get();
        run.set(n + 1);
        out.vertex([n as f32, 0.0, 0.0], [0.0, 1.0, 0.0]);
    })
    .panic_if_divergent();
}

#[test]
fn a_normal_divergence_is_distinguished_from_a_position_one() {
    let run = Cell::new(0u32);
    let report = check_determinism(|out: &mut MeshBuffer<f64>| {
        let n = run.get();
        run.set(n + 1);
        out.vertex([0.0, 0.0, 0.0], [0.0, if n == 0 { 1.0 } else { -1.0 }, 0.0]);
    });

    match report.divergence {
        Some((RunPair::RepeatedCall, Divergence::Normal { vertex, axis, .. })) => {
            assert_eq!((vertex, axis), (0, 1));
        }
        other => panic!("expected a normal divergence, got {other:?}"),
    }
}