tests: factor common HostMemory / MemArea code into a separate crate

crates/test/.gitignore

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+target
+Cargo.lock

crates/test/Cargo.toml

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+[package]
+name = "wiggle-test"
+version = "0.1.0"
+authors = ["Pat Hickey <phickey@fastly.com>", "Jakub Konka <kubkon@jakubkonka.com>"]
+edition = "2018"
+
+[dependencies]
+wiggle-runtime = { path = "../runtime" }
+proptest = "0.9"

crates/test/src/lib.rs

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+use proptest::prelude::*;
+use wiggle_runtime::GuestMemory;
+
+#[repr(align(4096))]
+pub struct HostMemory {
+    buffer: [u8; 4096],
+}
+impl HostMemory {
+    pub fn new() -> Self {
+        HostMemory { buffer: [0; 4096] }
+    }
+
+    pub fn guest_memory<'a>(&'a mut self) -> GuestMemory<'a> {
+        GuestMemory::new(self.buffer.as_mut_ptr(), self.buffer.len() as u32)
+    }
+
+    pub fn mem_area_strat(align: u32) -> BoxedStrategy<MemArea> {
+        prop::num::u32::ANY
+            .prop_filter_map("needs to fit in memory", move |p| {
+                let p_aligned = p - (p % align); // Align according to argument
+                let ptr = p_aligned % 4096; // Put inside memory
+                if ptr + align < 4096 {
+                    Some(MemArea { ptr, len: align })
+                } else {
+                    None
+                }
+            })
+            .boxed()
+    }
+}
+
+#[derive(Debug)]
+pub struct MemArea {
+    pub ptr: u32,
+    pub len: u32,
+}
+
+impl MemArea {
+    // This code is a whole lot like the Region::overlaps func thats at the core of the code under
+    // test.
+    // So, I implemented this one with std::ops::Range so it is less likely I wrote the same bug in two
+    // places.
+    pub fn overlapping(&self, b: &Self) -> bool {
+        // a_range is all elems in A
+        let a_range = std::ops::Range {
+            start: self.ptr,
+            end: self.ptr + self.len, // std::ops::Range is open from the right
+        };
+        // b_range is all elems in B
+        let b_range = std::ops::Range {
+            start: b.ptr,
+            end: b.ptr + b.len,
+        };
+        // No element in B is contained in A:
+        for b_elem in b_range.clone() {
+            if a_range.contains(&b_elem) {
+                return true;
+            }
+        }
+        // No element in A is contained in B:
+        for a_elem in a_range {
+            if b_range.contains(&a_elem) {
+                return true;
+            }
+        }
+        return false;
+    }
+    pub fn non_overlapping_set(areas: &[&Self]) -> bool {
+        // A is all areas
+        for (i, a) in areas.iter().enumerate() {
+            // (A, B) is every pair of areas
+            for b in areas[i + 1..].iter() {
+                if a.overlapping(b) {
+                    return false;
+                }
+            }
+        }
+        return true;
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    #[test]
+    fn hostmemory_is_aligned() {
+        let mut h = HostMemory::new();
+        assert_eq!(h.buffer.as_mut_ptr() as usize % 4096, 0);
+        let mut h = Box::new(HostMemory::new());
+        assert_eq!(h.buffer.as_mut_ptr() as usize % 4096, 0);
+    }
+}