Merging upstream version 1.71.1+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

8 files changed, 903 insertions, 0 deletions

diff --git a/vendor/varisat-internal-proof/.cargo-checksum.json b/vendor/varisat-internal-proof/.cargo-checksum.json
new file mode 100644
index 000000000..4de3cd320
--- /dev/null
+++ b/vendor/varisat-internal-proof/.cargo-checksum.json
@@ -0,0 +1 @@
+{"files":{"Cargo.toml":"3fc37c3493bb992e9d0362475a0fa2cb8bad821edc0f5274b18cc97597a8ccff","LICENSE-APACHE":"a60eea817514531668d7e00765731449fe14d059d3249e0bc93b36de45f759f2","LICENSE-MIT":"2259cbb177661977dd5510c05a8670da3cb28aa7f4ff74c193798db496fe92d8","README.md":"bfc5897a6a04d192e8bc3c56f64ae735271511a394ccc08c8ea1fe58f0168dfa","src/binary_format.rs":"76ff2dced33a22925107c91730bd19df8a58621519b9a5348fd85061c299677b","src/lib.rs":"80ce44fdae8f2e8125a950f51a2e03d352eb79beb3df7c5359fde3870656c0a8","src/vli_enc.rs":"23b3d52aa89623a3f4ede0a2bda39d6cf2b8ced5e73c3355b67ec33ff4842750"},"package":"6163bb7bc9018af077b76d64f976803d141c36a27d640f1437dddc4fd527d207"}
\ No newline at end of file
diff --git a/vendor/varisat-internal-proof/Cargo.toml b/vendor/varisat-internal-proof/Cargo.toml
new file mode 100644
index 000000000..27b1b87ff
--- /dev/null
+++ b/vendor/varisat-internal-proof/Cargo.toml
@@ -0,0 +1,29 @@
+# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g., crates.io) dependencies
+#
+# If you believe there's an error in this file please file an
+# issue against the rust-lang/cargo repository. If you're
+# editing this file be aware that the upstream Cargo.toml
+# will likely look very different (and much more reasonable)
+
+[package]
+edition = "2018"
+name = "varisat-internal-proof"
+version = "0.2.2"
+authors = ["Jannis Harder <me@jix.one>"]
+description = "Internal proof format for the Varisat SAT solver"
+homepage = "https://jix.one/project/varisat/"
+readme = "README.md"
+license = "MIT/Apache-2.0"
+repository = "https://github.com/jix/varisat"
+[dependencies.anyhow]
+version = "1.0.32"
+
+[dependencies.varisat-formula]
+version = "=0.2.2"
+[dev-dependencies.proptest]
+version = "0.10.1"
diff --git a/vendor/varisat-internal-proof/LICENSE-APACHE b/vendor/varisat-internal-proof/LICENSE-APACHE
new file mode 100644
index 000000000..16fe87b06
--- /dev/null
+++ b/vendor/varisat-internal-proof/LICENSE-APACHE
@@ -0,0 +1,201 @@
+                              Apache License
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+TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
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+Copyright [yyyy] [name of copyright owner]
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+	http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
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diff --git a/vendor/varisat-internal-proof/LICENSE-MIT b/vendor/varisat-internal-proof/LICENSE-MIT
new file mode 100644
index 000000000..4fd1cb2ff
--- /dev/null
+++ b/vendor/varisat-internal-proof/LICENSE-MIT
@@ -0,0 +1,25 @@
+Copyright (c) 2017-2019 Jannis Harder
+
+Permission is hereby granted, free of charge, to any
+person obtaining a copy of this software and associated
+documentation files (the "Software"), to deal in the
+Software without restriction, including without
+limitation the rights to use, copy, modify, merge,
+publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software
+is furnished to do so, subject to the following
+conditions:
+
+The above copyright notice and this permission notice
+shall be included in all copies or substantial portions
+of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
+ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
+TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
+SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
+IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
diff --git a/vendor/varisat-internal-proof/README.md b/vendor/varisat-internal-proof/README.md
new file mode 100644
index 000000000..29552e8c6
--- /dev/null
+++ b/vendor/varisat-internal-proof/README.md
@@ -0,0 +1,27 @@
+# Varisat
+
+This crate defines the proof format used internally by the [`varisat`
+crate][crate-varisat].
+
+__This is an internal dependency of the Varisat SAT solver, and should not be
+used directly.__
+
+## License
+
+The Varisat source code is licensed under either of
+
+  * Apache License, Version 2.0
+    ([LICENSE-APACHE](LICENSE-APACHE) or
+    http://www.apache.org/licenses/LICENSE-2.0)
+  * MIT license
+    ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+at your option.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in Varisat by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.
+
+[crate-varisat]: https://crates.io/crates/varisat
diff --git a/vendor/varisat-internal-proof/src/binary_format.rs b/vendor/varisat-internal-proof/src/binary_format.rs
new file mode 100644
index 000000000..e296468d1
--- /dev/null
+++ b/vendor/varisat-internal-proof/src/binary_format.rs
@@ -0,0 +1,332 @@
+//! Binary format for varisat proofs.
+use std::io::{self, BufRead, Write};
+
+use anyhow::Error;
+
+use varisat_formula::{Lit, Var};
+
+use crate::vli_enc::{read_u64, write_u64};
+
+use super::{ClauseHash, DeleteClauseProof, ProofStep};
+
+macro_rules! step_codes {
+    ($counter:expr, $name:ident, ) => {
+        const $name: u64 = $counter;
+    };
+    ($counter:expr, $name:ident, $($names:ident),* ,) => {
+        const $name: u64 = $counter;
+        step_codes!($counter + 1, $($names),* ,);
+    };
+}
+
+step_codes!(
+    0,
+    CODE_SOLVER_VAR_NAME_UPDATE,
+    CODE_SOLVER_VAR_NAME_REMOVE,
+    CODE_USER_VAR_NAME_UPDATE,
+    CODE_USER_VAR_NAME_REMOVE,
+    CODE_DELETE_VAR,
+    CODE_CHANGE_SAMPLING_MODE_SAMPLE,
+    CODE_CHANGE_SAMPLING_MODE_WITNESS,
+    CODE_AT_CLAUSE_RED,
+    CODE_AT_CLAUSE_IRRED,
+    CODE_UNIT_CLAUSES,
+    CODE_DELETE_CLAUSE_REDUNDANT,
+    CODE_DELETE_CLAUSE_SIMPLIFIED,
+    CODE_DELETE_CLAUSE_SATISFIED,
+    CODE_CHANGE_HASH_BITS,
+    CODE_MODEL,
+    CODE_ADD_CLAUSE,
+    CODE_ASSUMPTIONS,
+    CODE_FAILED_ASSUMPTIONS,
+);
+
+// Using a random value here makes it unlikely that a corrupted proof will be silently truncated and
+// accepted
+const CODE_END: u64 = 0x9ac3391f4294c211;
+
+/// Writes a proof step in the varisat format
+pub fn write_step<'s>(target: &mut impl Write, step: &'s ProofStep<'s>) -> io::Result<()> {
+    match *step {
+        ProofStep::SolverVarName { global, solver } => {
+            if let Some(solver) = solver {
+                write_u64(&mut *target, CODE_SOLVER_VAR_NAME_UPDATE)?;
+                write_u64(&mut *target, global.index() as u64)?;
+                write_u64(&mut *target, solver.index() as u64)?;
+            } else {
+                write_u64(&mut *target, CODE_SOLVER_VAR_NAME_REMOVE)?;
+                write_u64(&mut *target, global.index() as u64)?;
+            }
+        }
+
+        ProofStep::UserVarName { global, user } => {
+            if let Some(user) = user {
+                write_u64(&mut *target, CODE_USER_VAR_NAME_UPDATE)?;
+                write_u64(&mut *target, global.index() as u64)?;
+                write_u64(&mut *target, user.index() as u64)?;
+            } else {
+                write_u64(&mut *target, CODE_USER_VAR_NAME_REMOVE)?;
+                write_u64(&mut *target, global.index() as u64)?;
+            }
+        }
+
+        ProofStep::DeleteVar { var } => {
+            write_u64(&mut *target, CODE_DELETE_VAR)?;
+            write_u64(&mut *target, var.index() as u64)?;
+        }
+
+        ProofStep::ChangeSamplingMode { var, sample } => {
+            if sample {
+                write_u64(&mut *target, CODE_CHANGE_SAMPLING_MODE_SAMPLE)?;
+            } else {
+                write_u64(&mut *target, CODE_CHANGE_SAMPLING_MODE_WITNESS)?;
+            }
+            write_u64(&mut *target, var.index() as u64)?;
+        }
+
+        ProofStep::AddClause { clause } => {
+            write_u64(&mut *target, CODE_ADD_CLAUSE)?;
+            write_literals(&mut *target, clause)?;
+        }
+
+        ProofStep::AtClause {
+            redundant,
+            clause,
+            propagation_hashes,
+        } => {
+            if redundant {
+                write_u64(&mut *target, CODE_AT_CLAUSE_RED)?;
+            } else {
+                write_u64(&mut *target, CODE_AT_CLAUSE_IRRED)?;
+            }
+            write_literals(&mut *target, clause)?;
+            write_hashes(&mut *target, propagation_hashes)?;
+        }
+
+        ProofStep::UnitClauses { units } => {
+            write_u64(&mut *target, CODE_UNIT_CLAUSES)?;
+            write_unit_clauses(&mut *target, units)?;
+        }
+
+        ProofStep::DeleteClause { clause, proof } => {
+            match proof {
+                DeleteClauseProof::Redundant => {
+                    write_u64(&mut *target, CODE_DELETE_CLAUSE_REDUNDANT)?;
+                }
+                DeleteClauseProof::Simplified => {
+                    write_u64(&mut *target, CODE_DELETE_CLAUSE_SIMPLIFIED)?;
+                }
+                DeleteClauseProof::Satisfied => {
+                    write_u64(&mut *target, CODE_DELETE_CLAUSE_SATISFIED)?;
+                }
+            }
+
+            write_literals(&mut *target, clause)?;
+        }
+
+        ProofStep::ChangeHashBits { bits } => {
+            write_u64(&mut *target, CODE_CHANGE_HASH_BITS)?;
+            write_u64(&mut *target, bits as u64)?;
+        }
+
+        ProofStep::Model { assignment } => {
+            write_u64(&mut *target, CODE_MODEL)?;
+            write_literals(&mut *target, assignment)?;
+        }
+
+        ProofStep::Assumptions { assumptions } => {
+            write_u64(&mut *target, CODE_ASSUMPTIONS)?;
+            write_literals(&mut *target, assumptions)?;
+        }
+
+        ProofStep::FailedAssumptions {
+            failed_core,
+            propagation_hashes,
+        } => {
+            write_u64(&mut *target, CODE_FAILED_ASSUMPTIONS)?;
+            write_literals(&mut *target, failed_core)?;
+            write_hashes(&mut *target, propagation_hashes)?;
+        }
+
+        ProofStep::End => {
+            write_u64(&mut *target, CODE_END)?;
+        }
+    }
+
+    Ok(())
+}
+
+#[derive(Default)]
+pub struct Parser {
+    lit_buf: Vec<Lit>,
+    hash_buf: Vec<ClauseHash>,
+    unit_buf: Vec<(Lit, ClauseHash)>,
+}
+
+impl Parser {
+    pub fn parse_step<'a>(&'a mut self, source: &mut impl BufRead) -> Result<ProofStep<'a>, Error> {
+        let code = read_u64(&mut *source)?;
+        match code {
+            CODE_SOLVER_VAR_NAME_UPDATE => {
+                let global = Var::from_index(read_u64(&mut *source)? as usize);
+                let solver = Some(Var::from_index(read_u64(&mut *source)? as usize));
+                Ok(ProofStep::SolverVarName { global, solver })
+            }
+            CODE_SOLVER_VAR_NAME_REMOVE => {
+                let global = Var::from_index(read_u64(&mut *source)? as usize);
+                Ok(ProofStep::SolverVarName {
+                    global,
+                    solver: None,
+                })
+            }
+            CODE_USER_VAR_NAME_UPDATE => {
+                let global = Var::from_index(read_u64(&mut *source)? as usize);
+                let user = Some(Var::from_index(read_u64(&mut *source)? as usize));
+                Ok(ProofStep::UserVarName { global, user })
+            }
+            CODE_USER_VAR_NAME_REMOVE => {
+                let global = Var::from_index(read_u64(&mut *source)? as usize);
+                Ok(ProofStep::UserVarName { global, user: None })
+            }
+            CODE_DELETE_VAR => {
+                let var = Var::from_index(read_u64(&mut *source)? as usize);
+                Ok(ProofStep::DeleteVar { var })
+            }
+            CODE_CHANGE_SAMPLING_MODE_SAMPLE | CODE_CHANGE_SAMPLING_MODE_WITNESS => {
+                let var = Var::from_index(read_u64(&mut *source)? as usize);
+                Ok(ProofStep::ChangeSamplingMode {
+                    var,
+                    sample: code == CODE_CHANGE_SAMPLING_MODE_SAMPLE,
+                })
+            }
+            CODE_ADD_CLAUSE => {
+                read_literals(&mut *source, &mut self.lit_buf)?;
+                Ok(ProofStep::AddClause {
+                    clause: &self.lit_buf,
+                })
+            }
+            CODE_AT_CLAUSE_IRRED | CODE_AT_CLAUSE_RED => {
+                read_literals(&mut *source, &mut self.lit_buf)?;
+                read_hashes(&mut *source, &mut self.hash_buf)?;
+                Ok(ProofStep::AtClause {
+                    redundant: code == CODE_AT_CLAUSE_RED,
+                    clause: &self.lit_buf,
+                    propagation_hashes: &self.hash_buf,
+                })
+            }
+            CODE_UNIT_CLAUSES => {
+                read_unit_clauses(&mut *source, &mut self.unit_buf)?;
+                Ok(ProofStep::UnitClauses {
+                    units: &self.unit_buf,
+                })
+            }
+            CODE_DELETE_CLAUSE_REDUNDANT
+            | CODE_DELETE_CLAUSE_SIMPLIFIED
+            | CODE_DELETE_CLAUSE_SATISFIED => {
+                let proof = match code {
+                    CODE_DELETE_CLAUSE_REDUNDANT => DeleteClauseProof::Redundant,
+                    CODE_DELETE_CLAUSE_SIMPLIFIED => DeleteClauseProof::Simplified,
+                    CODE_DELETE_CLAUSE_SATISFIED => DeleteClauseProof::Satisfied,
+                    _ => unreachable!(),
+                };
+                read_literals(&mut *source, &mut self.lit_buf)?;
+                Ok(ProofStep::DeleteClause {
+                    clause: &self.lit_buf,
+                    proof,
+                })
+            }
+            CODE_CHANGE_HASH_BITS => {
+                let bits = read_u64(&mut *source)? as u32;
+                Ok(ProofStep::ChangeHashBits { bits })
+            }
+            CODE_MODEL => {
+                read_literals(&mut *source, &mut self.lit_buf)?;
+                Ok(ProofStep::Model {
+                    assignment: &self.lit_buf,
+                })
+            }
+            CODE_ASSUMPTIONS => {
+                read_literals(&mut *source, &mut self.lit_buf)?;
+                Ok(ProofStep::Assumptions {
+                    assumptions: &self.lit_buf,
+                })
+            }
+            CODE_FAILED_ASSUMPTIONS => {
+                read_literals(&mut *source, &mut self.lit_buf)?;
+                read_hashes(&mut *source, &mut self.hash_buf)?;
+                Ok(ProofStep::FailedAssumptions {
+                    failed_core: &self.lit_buf,
+                    propagation_hashes: &self.hash_buf,
+                })
+            }
+            CODE_END => Ok(ProofStep::End),
+            _ => anyhow::bail!("parse error"),
+        }
+    }
+}
+
+/// Writes a slice of literals for a varisat proof
+fn write_literals(target: &mut impl Write, literals: &[Lit]) -> io::Result<()> {
+    write_u64(&mut *target, literals.len() as u64)?;
+    for &lit in literals {
+        write_u64(&mut *target, lit.code() as u64)?;
+    }
+    Ok(())
+}
+
+/// Read a slice of literals from a varisat proof
+fn read_literals(source: &mut impl BufRead, literals: &mut Vec<Lit>) -> Result<(), io::Error> {
+    literals.clear();
+    let len = read_u64(&mut *source)? as usize;
+    literals.reserve(len);
+    for _ in 0..len {
+        literals.push(Lit::from_code(read_u64(&mut *source)? as usize));
+    }
+    Ok(())
+}
+
+/// Writes a slice of clause hashes for a varisat proof
+fn write_hashes(target: &mut impl Write, hashes: &[ClauseHash]) -> io::Result<()> {
+    write_u64(&mut *target, hashes.len() as u64)?;
+    for &hash in hashes {
+        write_u64(&mut *target, hash as u64)?;
+    }
+    Ok(())
+}
+
+/// Read a slice of clause hashes from a varisat proof
+fn read_hashes(source: &mut impl BufRead, hashes: &mut Vec<ClauseHash>) -> Result<(), io::Error> {
+    hashes.clear();
+    let len = read_u64(&mut *source)? as usize;
+    hashes.reserve(len);
+    for _ in 0..len {
+        hashes.push(read_u64(&mut *source)? as ClauseHash);
+    }
+    Ok(())
+}
+
+/// Writes a slice of unit clauses for a varisat proof
+fn write_unit_clauses(target: &mut impl Write, units: &[(Lit, ClauseHash)]) -> io::Result<()> {
+    write_u64(&mut *target, units.len() as u64)?;
+    for &(lit, hash) in units {
+        write_u64(&mut *target, lit.code() as u64)?;
+        write_u64(&mut *target, hash as u64)?;
+    }
+    Ok(())
+}
+
+/// Read a slice of unit clauses from a varisat proof
+fn read_unit_clauses(
+    source: &mut impl BufRead,
+    units: &mut Vec<(Lit, ClauseHash)>,
+) -> Result<(), io::Error> {
+    units.clear();
+    let len = read_u64(&mut *source)? as usize;
+    units.reserve(len);
+    for _ in 0..len {
+        let lit = Lit::from_code(read_u64(&mut *source)? as usize);
+        let hash = read_u64(&mut *source)? as ClauseHash;
+        units.push((lit, hash));
+    }
+    Ok(())
+}
diff --git a/vendor/varisat-internal-proof/src/lib.rs b/vendor/varisat-internal-proof/src/lib.rs
new file mode 100644
index 000000000..83d38dc5d
--- /dev/null
+++ b/vendor/varisat-internal-proof/src/lib.rs
@@ -0,0 +1,152 @@
+//! Internal proof format for the Varisat SAT solver.
+use varisat_formula::{Lit, Var};
+
+pub mod binary_format;
+
+mod vli_enc;
+
+// Integer type used to store a hash of a clause.
+pub type ClauseHash = u64;
+
+/// Hash a single literal.
+///
+/// Multiple literals can be combined with xor, as done in [`clause_hash`].
+pub fn lit_hash(lit: Lit) -> ClauseHash {
+    lit_code_hash(lit.code())
+}
+
+/// Hash a single literal from a code.
+///
+/// This doesn't require the code to correspond a valid literal.
+pub fn lit_code_hash(lit_code: usize) -> ClauseHash {
+    // Constant based on the golden ratio provides good mixing for the resulting upper bits
+    (!(lit_code as u64)).wrapping_mul(0x61c8864680b583ebu64)
+}
+
+/// A fast hash function for clauses (or other *sets* of literals).
+///
+/// This hash function interprets the given slice as a set and will not change when the input is
+/// permuted. It does not handle duplicated items.
+pub fn clause_hash(lits: &[Lit]) -> ClauseHash {
+    let mut hash = 0;
+    for &lit in lits {
+        hash ^= lit_hash(lit);
+    }
+    hash
+}
+
+/// Justifications for a simple clause deletion.
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub enum DeleteClauseProof {
+    /// The clause is known to be redundant.
+    Redundant,
+    /// The clause is irred and subsumed by the clause added in the previous step.
+    Simplified,
+    /// The clause contains a true literal.
+    ///
+    /// Also used to justify deletion of tautological clauses.
+    Satisfied,
+}
+
+/// A single proof step.
+///
+/// Represents a mutation of the current formula and a justification for the mutation's validity.
+#[derive(Copy, Clone, Debug)]
+pub enum ProofStep<'a> {
+    /// Update the global to solver var mapping.
+    ///
+    /// For proof checking, the solver variable names are only used for hash computations.
+    SolverVarName { global: Var, solver: Option<Var> },
+    /// Update the global to user var mapping.
+    ///
+    /// A variable without user mapping is considered hidden by the checker. When a variable without
+    /// user mapping gets a user mapping, the sampling mode is initialized to witness.
+    ///
+    /// It's not allowed to change a variable from one user name to another when the variable is in
+    /// use.
+    ///
+    /// Clause additions and assumptions are only allowed to use variables with user mappings (and a
+    /// non-witness sampling mode).
+    UserVarName { global: Var, user: Option<Var> },
+    /// Delete a variable.
+    ///
+    /// This is only allowed for variables that are isolated and hidden.
+    DeleteVar { var: Var },
+    /// Changes the sampling mode of a variable.
+    ///
+    /// This is only used to change between Sample and Witness. Hidden is managed by adding or
+    /// removing a user var name.
+    ChangeSamplingMode { var: Var, sample: bool },
+    /// Add a new input clause.
+    ///
+    /// This is only emitted for clauses added incrementally after an initial solve call.
+    AddClause { clause: &'a [Lit] },
+    /// Add a clause that is an asymmetric tautoligy (AT).
+    ///
+    /// Assuming the negation of the clause's literals leads to a unit propagation conflict.
+    ///
+    /// The second slice contains the hashes of all clauses involved in the resulting conflict. The
+    /// order of hashes is the order in which the clauses propagate when all literals of the clause
+    /// are set false.
+    ///
+    /// When generating DRAT proofs the second slice is ignored and may be empty.
+    AtClause {
+        redundant: bool,
+        clause: &'a [Lit],
+        propagation_hashes: &'a [ClauseHash],
+    },
+    /// Unit clauses found by top-level unit-propagation.
+    ///
+    /// Pairs of unit clauses and the original clause that became unit. Clauses are in chronological
+    /// order. This is equivalent to multiple `AtClause` steps where the clause is unit and the
+    /// propagation_hashes field contains just one hash, with the difference that this is not output
+    /// for DRAT proofs.
+    ///
+    /// Ignored when generating DRAT proofs.
+    UnitClauses { units: &'a [(Lit, ClauseHash)] },
+    /// Delete a clause consisting of the given literals.
+    DeleteClause {
+        clause: &'a [Lit],
+        proof: DeleteClauseProof,
+    },
+    /// Change the number of clause hash bits used
+    ChangeHashBits { bits: u32 },
+    /// A (partial) assignment that satisfies all clauses and assumptions.
+    Model { assignment: &'a [Lit] },
+    /// Change the active set of assumptions.
+    ///
+    /// This is checked against future model or failed assumptions steps.
+    Assumptions { assumptions: &'a [Lit] },
+    /// A subset of the assumptions that make the formula unsat.
+    FailedAssumptions {
+        failed_core: &'a [Lit],
+        propagation_hashes: &'a [ClauseHash],
+    },
+    /// Signals the end of a proof.
+    ///
+    /// A varisat proof must end with this command or else the checker will complain about an
+    /// incomplete proof.
+    End,
+}
+
+impl<'a> ProofStep<'a> {
+    /// Does this proof step use clause hashes?
+    pub fn contains_hashes(&self) -> bool {
+        match self {
+            ProofStep::AtClause { .. }
+            | ProofStep::UnitClauses { .. }
+            | ProofStep::FailedAssumptions { .. } => true,
+
+            ProofStep::SolverVarName { .. }
+            | ProofStep::UserVarName { .. }
+            | ProofStep::DeleteVar { .. }
+            | ProofStep::ChangeSamplingMode { .. }
+            | ProofStep::AddClause { .. }
+            | ProofStep::DeleteClause { .. }
+            | ProofStep::ChangeHashBits { .. }
+            | ProofStep::Model { .. }
+            | ProofStep::Assumptions { .. }
+            | ProofStep::End => false,
+        }
+    }
+}
diff --git a/vendor/varisat-internal-proof/src/vli_enc.rs b/vendor/varisat-internal-proof/src/vli_enc.rs
new file mode 100644
index 000000000..058058e6e
--- /dev/null
+++ b/vendor/varisat-internal-proof/src/vli_enc.rs
@@ -0,0 +1,136 @@
+//! Variable length integer encoding.
+//!
+//! This uses a different encoding from the leb128 encoding used for binary DRAT and CLRAT. It uses
+//! the same amount of bytes for each input, but is faster to encode and decode.
+//!
+//! Numbers are encoded like this:
+//! ```text
+//! 1xxxxxxx for up to 7 bits
+//! 01xxxxxx xxxxxxxx for up to 14 bits
+//! 001xxxxx xxxxxxxx xxxxxxxx for up to 21 bits
+//! ...
+//! ```
+//!
+//! The x-bits store the number from LSB to MSB. This scheme allows faster encoding and decoding, as
+//! the bits are kept consecutive and the length can be determined from the first or first two
+//! bytes.
+//!
+//! The current implementations are not optimal but fast enough to not be the bottleneck when
+//! checking proofs.
+use std::{
+    convert::{TryFrom, TryInto},
+    io::{self, BufRead, Write},
+};
+
+/// Write an encoded 64 bit number.
+pub fn write_u64(target: &mut impl Write, mut value: u64) -> Result<(), io::Error> {
+    let bits = (64 - value.leading_zeros()) as u32;
+    let blocks = (bits * (64 / 7)) / 64;
+    if value < (1 << (8 * 7)) {
+        value = ((value << 1) | 1) << blocks;
+        let bytes = unsafe { std::mem::transmute::<u64, [u8; 8]>(value.to_le()) };
+        target.write_all(&bytes[..(blocks + 1) as usize])
+    } else {
+        let lo_data = ((value << 1) | 1) << blocks;
+        let lo_bytes = unsafe { std::mem::transmute::<u64, [u8; 8]>(lo_data.to_le()) };
+        let hi_data = value >> (64 - (blocks + 1));
+        let hi_bytes = unsafe { std::mem::transmute::<u64, [u8; 8]>(hi_data.to_le()) };
+
+        target.write_all(&lo_bytes)?;
+        target.write_all(&hi_bytes[..(blocks as usize) + 1 - 8])
+    }
+}
+
+/// Read an encoded 64 bit number, if at least 16 bytes lookahead are available.
+fn read_u64_fast(bytes: &[u8; 16]) -> (u64, usize) {
+    let lo_data = u64::from_le(unsafe {
+        std::mem::transmute::<[u8; 8], u64>(*<&[u8; 8]>::try_from(&bytes[..8]).unwrap())
+    });
+
+    let len = (lo_data | (1 << 9)).trailing_zeros() + 1;
+
+    if len <= 8 {
+        let result = lo_data & (!0u64 >> (64 - 8 * len));
+
+        let result = result >> len;
+
+        (result, len as usize)
+    } else {
+        let hi_data = u64::from_le(unsafe {
+            std::mem::transmute::<[u8; 8], u64>(*<&[u8; 8]>::try_from(&bytes[8..]).unwrap())
+        });
+
+        let hi_data = hi_data & (!0u64 >> (64 - 8 * (len - 8)));
+
+        let result = (lo_data >> len) | (hi_data << (64 - len));
+
+        (result as u64, len as usize)
+    }
+}
+
+/// Read an encoded 64 bit number from a buffered reader.
+///
+/// This uses [`read_u64_fast`] if the remaining buffer is larger than 16 bytes and falls back to a
+/// slower implementation otherwise.
+pub fn read_u64(source: &mut impl BufRead) -> Result<u64, io::Error> {
+    let buf = source.fill_buf()?;
+    if buf.len() >= 16 {
+        let next_bytes: &[u8; 16] = (&buf[..16]).try_into().unwrap();
+        let (value, advance) = read_u64_fast(next_bytes);
+        source.consume(advance);
+        Ok(value)
+    } else {
+        let mut scan = 1 << 9;
+        let mut byte: [u8; 1] = [0];
+        let mut begin = 1;
+        source.read_exact(&mut byte[..])?;
+        let mut result = byte[0] as u64;
+        scan |= byte[0] as u64;
+        if byte[0] == 0 {
+            begin += 1;
+            source.read_exact(&mut byte[..])?;
+            result |= (byte[0] as u64) << 8;
+            scan |= (byte[0] as u64) << 8;
+        }
+        let len = scan.trailing_zeros() + 1;
+
+        result >>= len;
+
+        for i in begin..len {
+            source.read_exact(&mut byte[..])?;
+            result |= (byte[0] as u64) << (8 * i - len);
+        }
+
+        Ok(result as u64)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use proptest::prelude::*;
+
+    proptest! {
+        #[test]
+        fn roundtrip (
+            numbers in prop::collection::vec(prop::num::u64::ANY, 0..10_000)
+        ) {
+            let mut buf = vec![];
+
+            for &num in numbers.iter() {
+                write_u64(&mut buf, num)?;
+            }
+
+            let mut read = std::io::BufReader::with_capacity(128, &buf[..]);
+
+            let mut out = vec![];
+
+            while let Ok(num) = read_u64(&mut read) {
+                out.push(num)
+            }
+
+            prop_assert_eq!(numbers, out);
+        }
+    }
+}