From 306f096c64caf1a6ba6439311367c96c92934d42 Mon Sep 17 00:00:00 2001
From: Jack Wrenn <me@jswrenn.com>
Date: Tue, 9 Apr 2019 14:39:36 -0400
Subject: [PATCH] prevent constructing `MatrixSliceMutMN` for which disjoint
 indices may alias the same linear index

Fixes #486.
---
 Cargo.toml                     |  1 +
 src/base/construction_slice.rs | 32 ++++++++++++++++++++++++++++++++
 src/lib.rs                     |  1 +
 3 files changed, 34 insertions(+)

diff --git a/Cargo.toml b/Cargo.toml
index 120d2916..4371df05 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -37,6 +37,7 @@ generic-array  = "0.12"
 rand           = { version = "0.6", default-features = false }
 num-traits     = { version = "0.2", default-features = false }
 num-complex    = { version = "0.2", default-features = false }
+num-rational   = { version = "0.2", default-features = false }
 approx         = { version = "0.3", default-features = false }
 alga           = { version = "0.9", default-features = false }
 matrixmultiply = { version = "0.2", optional = true }
diff --git a/src/base/construction_slice.rs b/src/base/construction_slice.rs
index 0946653a..e5f311f8 100644
--- a/src/base/construction_slice.rs
+++ b/src/base/construction_slice.rs
@@ -2,6 +2,7 @@ use crate::base::dimension::{Dim, DimName, Dynamic, U1};
 use crate::base::matrix_slice::{SliceStorage, SliceStorageMut};
 use crate::base::{MatrixSliceMN, MatrixSliceMutMN, Scalar};
 
+use num_rational::Ratio;
 /*
  *
  * Slice constructors.
@@ -107,6 +108,37 @@ impl<'a, N: Scalar, R: Dim, C: Dim, RStride: Dim, CStride: Dim>
             "Matrix slice: input data buffer to small."
         );
 
+        assert!({
+            let nrows = nrows.value();
+            let ncols = ncols.value();
+            let rstride = rstride.value();
+            let cstride = cstride.value();
+
+            // `Storage::linear_index`
+            let index = |i, j| (i * rstride) + (j * cstride);
+
+            // The final condition of each arm is an expression in the form:
+            //    index(i₀, j₀) != index(i₁, j₁)
+            // If this expression is `false`, then the values (i₀, j₀)
+            // and (i₁, j₁) are concrete examples of indices that would
+            // collide if the matrix was actually constructed. 
+            nrows * ncols <= 1 ||
+                match (rstride, cstride) {
+                    (0, 0) => index(0, 0) != index(nrows - 1, ncols - 1),
+                    (0, _) => nrows <= 1 || index(0, 0) != index(nrows - 1, 0),
+                    (_, 0) => ncols <= 1 || index(0, 0) != index(0, ncols - 1),
+                    (_, _) => {
+                        let ratio = Ratio::new(rstride, cstride);
+                        let numer = *ratio.numer();
+                        let denom = *ratio.denom();
+
+                        nrows <= denom || ncols <= numer
+                            || index(0, numer) != index(denom, 0)
+                    }
+                }
+            },
+            "Matrix slice: dimensions and strides result in aliased indices.");
+
         unsafe {
             Self::from_slice_with_strides_generic_unchecked(data, 0, nrows, ncols, rstride, cstride)
         }
diff --git a/src/lib.rs b/src/lib.rs
index 1cab3da8..2c649f63 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -112,6 +112,7 @@ extern crate generic_array;
 extern crate matrixmultiply;
 extern crate num_complex;
 extern crate num_traits as num;
+extern crate num_rational;
 extern crate rand;
 extern crate typenum;