Add median-time-past (MTP) calculation to CheckPoint

crates/core/src/checkpoint.rs

@@ -2,6 +2,7 @@ use core::fmt;
 use core::ops::RangeBounds;
 
 use alloc::sync::Arc;
+use alloc::vec::Vec;
 use bitcoin::{block::Header, BlockHash};
 
 use crate::BlockId;
@@ -60,14 +61,22 @@ impl<D> Drop for CPInner<D> {
 
 /// Trait that converts [`CheckPoint`] `data` to [`BlockHash`].
 ///
-/// Implementations of [`ToBlockHash`] must always return the block’s consensus-defined hash. If
+/// Implementations of [`ToBlockHash`] must always return the block's consensus-defined hash. If
 /// your type contains extra fields (timestamps, metadata, etc.), these must be ignored. For
 /// example, [`BlockHash`] trivially returns itself, [`Header`] calls its `block_hash()`, and a
-/// wrapper type around a [`Header`] should delegate to the header’s hash rather than derive one
+/// wrapper type around a [`Header`] should delegate to the header's hash rather than derive one
 /// from other fields.
 pub trait ToBlockHash {
     /// Returns the [`BlockHash`] for the associated [`CheckPoint`] `data` type.
     fn to_blockhash(&self) -> BlockHash;
+
+    /// Returns the block time if available for the data type.
+    ///
+    /// Default implementation returns `None`. Data types that include timestamp information
+    /// (such as [`Header`]) should override this method.
+    fn block_time(&self) -> Option<u32> {
+        None
+    }
 }
 
 impl ToBlockHash for BlockHash {
@@ -80,6 +89,10 @@ impl ToBlockHash for Header {
     fn to_blockhash(&self) -> BlockHash {
         self.block_hash()
     }
+
+    fn block_time(&self) -> Option<u32> {
+        Some(self.time)
+    }
 }
 
 impl<D> PartialEq for CheckPoint<D> {
@@ -195,6 +208,8 @@ impl<D> CheckPoint<D>
 where
     D: ToBlockHash + fmt::Debug + Copy,
 {
+    const MTP_BLOCK_COUNT: u32 = 11;
+
     /// Construct a new base [`CheckPoint`] from given `height` and `data` at the front of a linked
     /// list.
     pub fn new(height: u32, data: D) -> Self {
@@ -208,6 +223,54 @@ where
         }))
     }
 
+    /// Calculate the median time past (MTP) for this checkpoint.
+    ///
+    /// Uses the 11 previous blocks (heights h-11 through h-1, where h is the current height)
+    /// to compute the MTP for the current block. This is used in Bitcoin's consensus rules
+    /// for time-based validations (BIP113).
+    ///
+    /// Note: This is a pseudo-median that doesn't average the two middle values.
+    ///
+    /// Returns `None` if the data type doesn't support block times or if any of the required
+    /// 11 sequential blocks are missing.
+    pub fn median_time_past(&self) -> Option<u32> {
+        let current_height = self.height();
+        let earliest_height = current_height.saturating_sub(Self::MTP_BLOCK_COUNT);
+        self._median_time_past(earliest_height..current_height)
+    }
+
+    /// Calculate the median time past (MTP) for the next block.
+    ///
+    /// Uses the 11 most recent blocks (heights h-10 through h, where h is the current height)
+    /// to compute what the MTP would be if a new block were added at height h+1.
+    ///
+    /// This differs from [`median_time_past`] which uses blocks h-11 through h-1 to compute
+    /// the MTP for the current block at height h.
+    ///
+    /// Returns `None` if the data type doesn't support block times or if any of the required
+    /// 11 sequential blocks are missing.
+    ///
+    /// [`median_time_past`]: CheckPoint::median_time_past
+    pub fn next_median_time_past(&self) -> Option<u32> {
+        let current_height = self.height();
+        let earliest_height = current_height.saturating_sub(Self::MTP_BLOCK_COUNT - 1);
+        self._median_time_past(earliest_height..=current_height)
+    }
+
+    fn _median_time_past(&self, heights: impl IntoIterator<Item = u32>) -> Option<u32> {
+        let mut timestamps = heights
+            .into_iter()
+            .map(|height| {
+                // Return `None` for missing blocks or missing block times
+                let cp = self.get(height)?;
+                let block_time = cp.data_ref().block_time()?;
+                Some(block_time)
+            })
+            .collect::<Option<Vec<u32>>>()?;
+        timestamps.sort_unstable();
+        Some(timestamps[timestamps.len().checked_sub(1)? / 2])
+    }
+
     /// Construct from an iterator of block data.
     ///
     /// Returns `Err(None)` if `blocks` doesn't yield any data. If the blocks are not in ascending

crates/core/tests/test_checkpoint.rs

@@ -1,5 +1,6 @@
-use bdk_core::CheckPoint;
+use bdk_core::{CheckPoint, ToBlockHash};
 use bdk_testenv::{block_id, hash};
+use bitcoin::hashes::Hash;
 use bitcoin::BlockHash;
 
 /// Inserting a block that already exists in the checkpoint chain must always succeed.
@@ -55,3 +56,173 @@ fn checkpoint_destruction_is_sound() {
     }
     assert_eq!(cp.iter().count() as u32, end);
 }
+
+/// Test helper: A block data type that includes timestamp
+/// Fields are (height, time)
+#[derive(Debug, Clone, Copy)]
+struct BlockWithTime(u32, u32);
+
+impl ToBlockHash for BlockWithTime {
+    fn to_blockhash(&self) -> BlockHash {
+        // Generate a deterministic hash from the height
+        let hash_bytes = bitcoin::hashes::sha256d::Hash::hash(&self.0.to_le_bytes());
+        BlockHash::from_raw_hash(hash_bytes)
+    }
+
+    fn block_time(&self) -> Option<u32> {
+        Some(self.1)
+    }
+}
+
+#[test]
+fn test_median_time_past_with_no_timestamps_available() {
+    // Test with BlockHash (no timestamps available)
+    let blocks = vec![
+        (0, hash!("genesis")),
+        (1, hash!("A")),
+        (2, hash!("B")),
+        (3, hash!("C")),
+    ];
+
+    let cp = CheckPoint::<BlockHash>::from_blocks(blocks).expect("must construct valid chain");
+    assert_eq!(cp.median_time_past(), None, "BlockHash has no timestamp");
+}
+
+#[test]
+fn test_median_time_past_with_timestamps() {
+    // Create a chain with 12 blocks (heights 0-11) with incrementing timestamps
+    let blocks: Vec<_> = (0..=11)
+        .map(|i| (i, BlockWithTime(i, 1000 + i * 10)))
+        .collect();
+
+    let cp = CheckPoint::from_blocks(blocks).expect("must construct valid chain");
+
+    // Height 11: 11 previous blocks (10..0), pseudo-median at index 5 = 1050
+    assert_eq!(cp.median_time_past(), Some(1050));
+
+    // Height 10: 10 previous blocks (9..0), pseudo-median at index 4 = 1040
+    assert_eq!(cp.get(10).unwrap().median_time_past(), Some(1040));
+
+    // Height 5: 5 previous blocks (4..0), pseudo-median at index 2 = 1020
+    assert_eq!(cp.get(5).unwrap().median_time_past(), Some(1020));
+
+    // Height 3: 3 previous blocks (2..0), pseudo-median at index 1 = 1010
+    assert_eq!(cp.get(3).unwrap().median_time_past(), Some(1010));
+
+    // Height 0: genesis has no previous blocks
+    assert_eq!(cp.get(0).unwrap().median_time_past(), None);
+}
+
+#[test]
+fn test_both_mtp_methods_comparison() {
+    // Create a chain with 15 blocks to test both MTP methods
+    let blocks: Vec<_> = (0..=14)
+        .map(|i| (i, BlockWithTime(i, 1000 + i * 10)))
+        .collect();
+
+    let cp = CheckPoint::from_blocks(blocks).expect("must construct valid chain");
+
+    // At height 12:
+    // median_time_past: uses blocks 1-11, median of [1010..1110] = 1060
+    // next_median_time_past: uses blocks 2-12, median of [1020..1120] = 1070
+    let cp12 = cp.get(12).unwrap();
+    assert_eq!(cp12.median_time_past(), Some(1060));
+    assert_eq!(cp12.next_median_time_past(), Some(1070));
+
+    // At height 11:
+    // median_time_past: uses blocks 0-10, median of [1000..1100] = 1050
+    // next_median_time_past: uses blocks 1-11, median of [1010..1110] = 1060
+    let cp11 = cp.get(11).unwrap();
+    assert_eq!(cp11.median_time_past(), Some(1050));
+    assert_eq!(cp11.next_median_time_past(), Some(1060));
+
+    // Verify the relationship: cp(n).next_mtp == cp(n+1).mtp
+    assert_eq!(cp11.next_median_time_past(), cp12.median_time_past());
+}
+
+#[test]
+fn test_next_median_time_past_edge_cases() {
+    // Test with minimum required blocks (11)
+    let blocks: Vec<_> = (0..=10)
+        .map(|i| (i, BlockWithTime(i, 1000 + i * 100)))
+        .collect();
+
+    let cp = CheckPoint::from_blocks(blocks).expect("must construct valid chain");
+
+    // At height 10: next_mtp uses all 11 blocks (0-10)
+    // Times: [1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000]
+    // Median at index 5 = 1500
+    assert_eq!(cp.next_median_time_past(), Some(1500));
+
+    // At height 9: next_mtp uses blocks 0-9 (10 blocks)
+    // Times: [1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900]
+    // Median at index 4 = 1400
+    assert_eq!(cp.get(9).unwrap().next_median_time_past(), Some(1400));
+
+    // Test sparse chain where next_mtp returns None due to missing blocks
+    let sparse = vec![
+        (0, BlockWithTime(0, 1000)),
+        (5, BlockWithTime(5, 1050)),
+        (10, BlockWithTime(10, 1100)),
+    ];
+    let sparse_cp = CheckPoint::from_blocks(sparse).expect("must construct valid chain");
+
+    // At height 10: next_mtp needs blocks 0-10 but many are missing
+    assert_eq!(sparse_cp.next_median_time_past(), None);
+}
+
+#[test]
+fn test_mtp_with_non_monotonic_times() {
+    // Test both methods with shuffled timestamps
+    let blocks = vec![
+        (0, BlockWithTime(0, 1500)),
+        (1, BlockWithTime(1, 1200)),
+        (2, BlockWithTime(2, 1800)),
+        (3, BlockWithTime(3, 1100)),
+        (4, BlockWithTime(4, 1900)),
+        (5, BlockWithTime(5, 1300)),
+        (6, BlockWithTime(6, 1700)),
+        (7, BlockWithTime(7, 1400)),
+        (8, BlockWithTime(8, 1600)),
+        (9, BlockWithTime(9, 1000)),
+        (10, BlockWithTime(10, 2000)),
+        (11, BlockWithTime(11, 1050)),
+    ];
+
+    let cp = CheckPoint::from_blocks(blocks).expect("must construct valid chain");
+
+    // Height 11:
+    // median_time_past uses blocks 0-10: sorted
+    // [1000,1100,1200,1300,1400,1500,1600,1700,1800,1900,2000] Median at index 5 = 1500
+    assert_eq!(cp.median_time_past(), Some(1500));
+
+    // next_median_time_past uses blocks 1-11: sorted
+    // [1000,1050,1100,1200,1300,1400,1600,1700,1800,1900,2000] Median at index 5 = 1400
+    assert_eq!(cp.next_median_time_past(), Some(1400));
+
+    // Test with smaller chain to verify sorting at different heights
+    let cp4 = cp.get(4).unwrap();
+    // Height 4: previous times [1100, 1800, 1200, 1500] -> sorted [1100, 1200, 1500, 1800]
+    // Pseudo-median at index 1 = 1200
+    assert_eq!(cp4.median_time_past(), Some(1200));
+}
+
+#[test]
+fn test_mtp_sparse_chain_both_methods() {
+    // Sparse chain missing required sequential blocks
+    let blocks = vec![
+        (0, BlockWithTime(0, 1000)),
+        (3, BlockWithTime(3, 1030)),
+        (7, BlockWithTime(7, 1070)),
+        (11, BlockWithTime(11, 1110)),
+        (15, BlockWithTime(15, 1150)),
+    ];
+
+    let cp = CheckPoint::from_blocks(blocks).expect("must construct valid chain");
+
+    // All heights should return None due to missing sequential blocks
+    assert_eq!(cp.median_time_past(), None);
+    assert_eq!(cp.next_median_time_past(), None);
+    assert_eq!(cp.get(11).unwrap().median_time_past(), None);
+    assert_eq!(cp.get(11).unwrap().next_median_time_past(), None);
+}