Add a Threshold<T> type

We have various enums in the codebase that include a `Thresh` variant,
we have to explicitly check that invariants are maintained all over the
place because these enums are public (eg, `policy::Concrete`).

Add a `Threshold<T>` type that abstracts over a threshold and maintains
the following invariants:

- v.len() > 0
- k > 0
- k <= v.len()

Author: tcharding

src/lib.rs

@@ -126,6 +126,7 @@ pub mod miniscript;
 pub mod plan;
 pub mod policy;
 pub mod psbt;
+pub mod threshold;
 
 #[cfg(test)]
 mod test_utils;

src/threshold.rs

@@ -0,0 +1,83 @@
+// SPDX-License-Identifier: CC0-1.0
+
+//! A generic (k,n)-threshold type.
+
+use core::fmt;
+
+/// A (k, n)-threshold.
+///
+/// This type maintains the following invariants:
+/// -   n > 0
+/// -   k > 0
+/// -   k <= n
+#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct Threshold<T> {
+    k: usize,
+    v: Vec<T>,
+}
+
+impl<T> Threshold<T> {
+    /// Creates a `Theshold<T>` after checking that invariants hold.
+    pub fn new(k: usize, v: Vec<T>) -> Result<Threshold<T>, Error> {
+        if v.len() == 0 {
+            Err(Error::ZeroN)
+        } else if k == 0 {
+            Err(Error::ZeroK)
+        } else if k > v.len() {
+            Err(Error::BigK)
+        } else {
+            Ok(Threshold { k, v })
+        }
+    }
+
+    /// Creates a `Theshold<T>` without checking that invariants hold.
+    pub fn new_unchecked(k: usize, v: Vec<T>) -> Threshold<T> { Threshold { k, v } }
+
+    /// Returns `k`, the threshold value.
+    pub fn k(&self) -> usize { self.k }
+
+    /// Returns `n`, the total number of elements in the threshold.
+    pub fn n(&self) -> usize { self.v.len() }
+
+    /// Returns a read-only iterator over the threshold elements.
+    pub fn iter(&self) -> core::slice::Iter<'_, T> { self.v.iter() }
+
+    /// Returns the threshold elements, consuming self.
+    // TODO: Find a better name for this functiion.
+    pub fn into_elements(self) -> Vec<T> { self.v }
+}
+
+/// An error attempting to construct a `Threshold<T>`.
+#[derive(Debug, Clone, PartialEq, Eq)]
+#[non_exhaustive]
+pub enum Error {
+    /// Threshold `n` value must be non-zero.
+    ZeroN,
+    /// Threshold `k` value must be non-zero.
+    ZeroK,
+    /// Threshold `k` value must be <= `n`.
+    BigK,
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use Error::*;
+
+        match *self {
+            ZeroN => f.write_str("threshold `n` value must be non-zero"),
+            ZeroK => f.write_str("threshold `k` value must be non-zero"),
+            BigK => f.write_str("threshold `k` value must be <= `n`"),
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for Error {
+    fn cause(&self) -> Option<&dyn std::error::Error> {
+        use Error::*;
+
+        match *self {
+            ZeroN | ZeroK | BigK => None,
+        }
+    }
+}