1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158

// Copyright (c) Facebook, Inc. and its affiliates.
//
// This source code is licensed under the MIT license found in the
// LICENSE file in the root directory of this source tree.

use crate::{ProofOptions, TraceInfo, TraceLayout};
use math::StarkField;
use utils::{
    collections::Vec, string::ToString, ByteReader, ByteWriter, Deserializable,
    DeserializationError, Serializable,
};

// PROOF CONTEXT
// ================================================================================================
/// Basic metadata about a specific execution of a computation.
#[derive(Debug, Clone, Eq, PartialEq)]
pub struct Context {
    trace_layout: TraceLayout,
    trace_length: usize,
    trace_meta: Vec<u8>,
    field_modulus_bytes: Vec<u8>,
    options: ProofOptions,
}

impl Context {
    // CONSTRUCTOR
    // --------------------------------------------------------------------------------------------
    /// Creates a new context for a computation described by the specified field, trace info, and
    /// proof options.
    pub fn new<B: StarkField>(trace_info: &TraceInfo, options: ProofOptions) -> Self {
        Context {
            trace_layout: trace_info.layout().clone(),
            trace_length: trace_info.length(),
            trace_meta: trace_info.meta().to_vec(),
            field_modulus_bytes: B::get_modulus_le_bytes(),
            options,
        }
    }

    // PUBLIC ACCESSORS
    // --------------------------------------------------------------------------------------------

    /// Returns a layout describing how columns of the execution trace described by this context
    /// are arranged into segments.
    pub fn trace_layout(&self) -> &TraceLayout {
        &self.trace_layout
    }

    /// Returns execution trace length of the computation described by this context.
    pub fn trace_length(&self) -> usize {
        self.trace_length
    }

    /// Returns execution trace info for the computation described by this context.
    pub fn get_trace_info(&self) -> TraceInfo {
        TraceInfo::new_multi_segment(
            self.trace_layout.clone(),
            self.trace_length(),
            self.trace_meta.clone(),
        )
    }

    /// Returns the size of the LDE domain for the computation described by this context.
    pub fn lde_domain_size(&self) -> usize {
        self.trace_length() * self.options.blowup_factor()
    }

    /// Returns modulus of the field for the computation described by this context.
    pub fn field_modulus_bytes(&self) -> &[u8] {
        &self.field_modulus_bytes
    }

    /// Returns number of bits in the base field modulus for the computation described by this
    /// context.
    ///
    /// The modulus is assumed to be encoded in little-endian byte order.
    pub fn num_modulus_bits(&self) -> u32 {
        let mut num_bits = self.field_modulus_bytes.len() as u32 * 8;
        for &byte in self.field_modulus_bytes.iter().rev() {
            if byte != 0 {
                num_bits -= byte.leading_zeros();
                return num_bits;
            }
            num_bits -= 8;
        }

        0
    }

    /// Returns proof options which were used to a proof in this context.
    pub fn options(&self) -> &ProofOptions {
        &self.options
    }
}

impl Serializable for Context {
    /// Serializes `self` and writes the resulting bytes into the `target`.
    fn write_into<W: ByteWriter>(&self, target: &mut W) {
        self.trace_layout.write_into(target);
        target.write_u8(math::log2(self.trace_length) as u8); // store as power of two
        target.write_u16(self.trace_meta.len() as u16);
        target.write_u8_slice(&self.trace_meta);
        assert!(self.field_modulus_bytes.len() < u8::MAX as usize);
        target.write_u8(self.field_modulus_bytes.len() as u8);
        target.write_u8_slice(&self.field_modulus_bytes);
        self.options.write_into(target);
    }
}

impl Deserializable for Context {
    /// Reads proof context from the specified `source` and returns the result.
    ///
    /// # Errors
    /// Returns an error of a valid Context struct could not be read from the specified `source`.
    fn read_from<R: ByteReader>(source: &mut R) -> Result<Self, DeserializationError> {
        // read and validate trace layout info
        let trace_layout = TraceLayout::read_from(source)?;

        // read and validate trace length (which was stored as a power of two)
        let trace_length = source.read_u8()?;
        if trace_length < math::log2(TraceInfo::MIN_TRACE_LENGTH) as u8 {
            return Err(DeserializationError::InvalidValue(format!(
                "trace length cannot be smaller than 2^{}, but was 2^{}",
                math::log2(TraceInfo::MIN_TRACE_LENGTH),
                trace_length
            )));
        }
        let trace_length = 2_usize.pow(trace_length as u32);

        // read trace metadata
        let num_meta_bytes = source.read_u16()? as usize;
        let trace_meta = if num_meta_bytes != 0 {
            source.read_u8_vec(num_meta_bytes)?
        } else {
            vec![]
        };

        // read and validate field modulus bytes
        let num_modulus_bytes = source.read_u8()? as usize;
        if num_modulus_bytes == 0 {
            return Err(DeserializationError::InvalidValue(
                "field modulus cannot be an empty value".to_string(),
            ));
        }
        let field_modulus_bytes = source.read_u8_vec(num_modulus_bytes)?;

        // read options
        let options = ProofOptions::read_from(source)?;

        Ok(Context {
            trace_layout,
            trace_length,
            trace_meta,
            field_modulus_bytes,
            options,
        })
    }
}