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feature: Add an ExpandedDifficulty type and conversion 

Also add tests.
This commit is contained in:
teor 2020-07-30 21:11:42 +10:00
parent c4dec3fb36
commit 195948e5b1
3 changed files with 242 additions and 6 deletions
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2
Cargo.lock generated
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@ -2644,7 +2644,9 @@ dependencies = [
"serde",
"serde-big-array",
"sha2",
"spandoc",
"thiserror",
"tracing",
"x25519-dalek",
"zebra-test",
]

16
zebra-chain/Cargo.toml
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@ -22,19 +22,23 @@ rand_core = "0.5.1"
ripemd160 = "0.8.0"
secp256k1 = { version = "0.17.2", features = ["serde"] }
serde = { version = "1", features = ["serde_derive", "rc"] }
serde-big-array = "0.3.0"
sha2 = { version = "0.8.2", features=["compress"] }
thiserror = "1"
x25519-dalek = { version = "0.6", features = ["serde"] }
serde-big-array = "0.3.0"
# ZF deps
ed25519-zebra = "1.0"
redjubjub = "0.2"
equihash = "0.1"
displaydoc = "0.1.7"
ed25519-zebra = "1.0"
equihash = "0.1"
redjubjub = "0.2"
[dev-dependencies]
bincode = "1"
color-eyre = "0.5"
proptest = "0.10"
proptest-derive = "0.2.0"
spandoc = "0.2"
tracing = "0.1.17"
zebra-test = { path = "../zebra-test/" }
color-eyre = "0.5"
bincode = "1"

230
zebra-chain/src/block/difficulty.rs
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@ -21,6 +21,236 @@ use proptest_derive::Arbitrary;
/// - calculating the 256-bit `ExpandedDifficulty` threshold, for comparison
/// with the block header hash, and
/// - calculating the block work.
///
/// Details:
///
/// This is a floating-point encoding, with a 24-bit signed mantissa,
/// an 8-bit exponent, an offset of 3, and a radix of 256.
/// (IEEE 754 32-bit floating-point values use a separate sign bit, an implicit
/// leading mantissa bit, an offset of 127, and a radix of 2.)
#[derive(Clone, Copy, Debug, Eq, PartialEq, Serialize, Deserialize)]
#[cfg_attr(test, derive(Arbitrary))]
pub struct CompactDifficulty(pub u32);
/// A 256-bit expanded difficulty value.
///
/// Used as a target threshold for the difficulty of a `BlockHeaderHash`.
#[derive(Clone, Copy, Debug, Eq, PartialEq, Serialize, Deserialize)]
#[cfg_attr(test, derive(Arbitrary))]
pub struct ExpandedDifficulty([u8; 32]);
impl CompactDifficulty {
/// CompactDifficulty floating-point precision.
const PRECISION: u32 = 24;
/// CompactDifficulty sign bit, part of the signed mantissa.
const SIGN_BIT: u32 = 1 << (CompactDifficulty::PRECISION - 1);
/// CompactDifficulty unsigned mantissa mask.
///
/// Also the maximum unsigned mantissa value.
const U_MANT_MASK: u32 = CompactDifficulty::SIGN_BIT - 1;
/// CompactDifficulty exponent offset.
const OFFSET: i32 = 3;
/// Calculate the ExpandedDifficulty for a compact representation.
///
/// See `ToTarget()` in the Zcash Specification, and `CheckProofOfWork()` in
/// zcashd.
///
/// Returns None for negative, zero, and overflow values. (zcashd rejects
/// these values, before comparing the hash.)
pub fn to_expanded(&self) -> Option<ExpandedDifficulty> {
// The constants for this floating-point representation.
// Alias the struct constants here, so the code is easier to read.
const PRECISION: u32 = CompactDifficulty::PRECISION;
const SIGN_BIT: u32 = CompactDifficulty::SIGN_BIT;
const U_MANT_MASK: u32 = CompactDifficulty::U_MANT_MASK;
const OFFSET: i32 = CompactDifficulty::OFFSET;
// Negative values in this floating-point representation.
// 0 if (x & 2^23 == 2^23)
// zcashd rejects negative values without comparing the hash.
if self.0 & SIGN_BIT == SIGN_BIT {
return None;
}
// The components of the result
// The fractional part of the number
// x & (2^23 - 1)
let mantissa = self.0 & U_MANT_MASK;
// The position of the number in the result, in bytes (rather than bits)
// 256^(floor(x/(2^24)) - 3)
// The i32 conversion is safe, because we've just divided self by 2^24.
let exponent = ((self.0 >> PRECISION) as i32) - OFFSET;
// Now put the mantissa in the right place in the result, based on
// the exponent.
let mut result = [0; 32];
for (i, b) in mantissa.to_le_bytes().iter().enumerate() {
// These conversions are safe, due to the size of the array, and the
// range checks before array access.
let position = exponent + i as i32;
if position >= 32 {
if *b != 0 {
// zcashd rejects overflow values, without comparing the
// hash
return None;
}
} else if position >= 0 {
// zcashd truncates fractional values
result[position as usize] = *b;
}
}
if result == [0; 32] {
// zcashd rejects zero values, without comparing the hash
None
} else {
Some(ExpandedDifficulty(result))
}
}
}
#[cfg(test)]
mod tests {
use super::*;
use color_eyre::eyre::Report;
use std::sync::Arc;
use crate::block::{Block, BlockHeaderHash};
use crate::serialization::ZcashDeserialize;
// Alias the struct constants here, so the code is easier to read.
const PRECISION: u32 = CompactDifficulty::PRECISION;
const SIGN_BIT: u32 = CompactDifficulty::SIGN_BIT;
const U_MANT_MASK: u32 = CompactDifficulty::U_MANT_MASK;
const OFFSET: i32 = CompactDifficulty::OFFSET;
/// Test zero values for CompactDifficulty.
#[test]
fn compact_zero() {
zebra_test::init();
let natural_zero = CompactDifficulty(0);
assert_eq!(natural_zero.to_expanded(), None);
// Small value zeroes
let small_zero_1 = CompactDifficulty(1);
assert_eq!(small_zero_1.to_expanded(), None);
let small_zero_max = CompactDifficulty(U_MANT_MASK);
assert_eq!(small_zero_max.to_expanded(), None);
// Special-cased zeroes, negative in the floating-point representation
let sc_zero = CompactDifficulty(SIGN_BIT);
assert_eq!(sc_zero.to_expanded(), None);
let sc_zero_next = CompactDifficulty(SIGN_BIT + 1);
assert_eq!(sc_zero_next.to_expanded(), None);
let sc_zero_high = CompactDifficulty((1 << PRECISION) - 1);
assert_eq!(sc_zero_high.to_expanded(), None);
let sc_zero_max = CompactDifficulty(u32::MAX);
assert_eq!(sc_zero_max.to_expanded(), None);
}
/// Test extreme values for CompactDifficulty.
#[test]
fn compact_extremes() {
zebra_test::init();
// Values equal to one
let mut expanded_one = [0; 32];
expanded_one[0] = 1;
let expanded_one = Some(ExpandedDifficulty(expanded_one));
let one = CompactDifficulty(OFFSET as u32 * (1 << PRECISION) + 1);
assert_eq!(one.to_expanded(), expanded_one);
let another_one = CompactDifficulty((1 << PRECISION) + (1 << 16));
assert_eq!(another_one.to_expanded(), expanded_one);
// Maximum mantissa
let mut expanded_mant = [0; 32];
expanded_mant[0] = 0xff;
expanded_mant[1] = 0xff;
expanded_mant[2] = 0x7f;
let expanded_mant = Some(ExpandedDifficulty(expanded_mant));
let mant = CompactDifficulty(OFFSET as u32 * (1 << PRECISION) + U_MANT_MASK);
assert_eq!(mant.to_expanded(), expanded_mant);
// Maximum valid exponent
let mut expanded_exp = [0; 32];
expanded_exp[31] = 1;
let expanded_exp = Some(ExpandedDifficulty(expanded_exp));
let exp = CompactDifficulty((31 + OFFSET as u32) * (1 << PRECISION) + 1);
assert_eq!(exp.to_expanded(), expanded_exp);
// Maximum valid mantissa and exponent
let mut expanded_me = [0; 32];
expanded_me[29] = 0xff;
expanded_me[30] = 0xff;
expanded_me[31] = 0x7f;
let expanded_me = Some(ExpandedDifficulty(expanded_me));
let me = CompactDifficulty((31 + 1) * (1 << PRECISION) + U_MANT_MASK);
assert_eq!(me.to_expanded(), expanded_me);
// Maximum value, at least according to the spec
//
// According to ToTarget() in the spec, this value is
// `(2^23 - 1) * 256^253`, which is larger than the maximum expanded
// value. Therefore, a block can never pass with this threshold.
//
// zcashd rejects these blocks without comparing the hash.
let difficulty_max = CompactDifficulty(u32::MAX & !SIGN_BIT);
assert_eq!(difficulty_max.to_expanded(), None);
}
/// Test blocks using CompactDifficulty.
#[test]
#[spandoc::spandoc]
fn compact_blocks() -> Result<(), Report> {
zebra_test::init();
let mut blockchain = Vec::new();
for b in &[
&zebra_test::vectors::BLOCK_MAINNET_GENESIS_BYTES[..],
&zebra_test::vectors::BLOCK_MAINNET_1_BYTES[..],
&zebra_test::vectors::BLOCK_MAINNET_2_BYTES[..],
&zebra_test::vectors::BLOCK_MAINNET_3_BYTES[..],
&zebra_test::vectors::BLOCK_MAINNET_4_BYTES[..],
&zebra_test::vectors::BLOCK_MAINNET_5_BYTES[..],
&zebra_test::vectors::BLOCK_MAINNET_6_BYTES[..],
&zebra_test::vectors::BLOCK_MAINNET_7_BYTES[..],
&zebra_test::vectors::BLOCK_MAINNET_8_BYTES[..],
&zebra_test::vectors::BLOCK_MAINNET_9_BYTES[..],
&zebra_test::vectors::BLOCK_MAINNET_10_BYTES[..],
] {
let block = Arc::<Block>::zcash_deserialize(*b)?;
let hash: BlockHeaderHash = block.as_ref().into();
blockchain.push((block.clone(), block.coinbase_height().unwrap(), hash));
}
// Now verify each block
for (block, height, hash) in blockchain {
/// SPANDOC: Check the difficulty of a mainnet block {?height, ?hash}
let threshold = block
.header
.difficulty_threshold
.to_expanded()
.expect("Chain blocks have valid difficulty thresholds.");
// Check the difficulty of the block.
//
// Invert the "less than or equal" comparison, because we interpret
// these values in little-endian order.
// TODO: replace with PartialOrd implementation
assert!(hash.0 >= threshold.0);
}
Ok(())
}
}