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|
// -*- rust -*-
/*
Module: tri
ADT for the ternary Kleene logic K3
This allows reasoning with three logic values (true, false, unknown).
Implementation: Truth values are represented using a single u8 and
all operations are done using bit operations which is fast
on current cpus.
*/
export t, true, false, unknown;
export not, and, or, xor, implies, eq, ne, is_true, is_false;
export from_str, to_str, all_values, to_bit;
/*
Type: t
The type of ternary logic values
*/
type t = u8;
const b0: u8 = 1u8;
const b1: u8 = 2u8;
const b01: u8 = 3u8;
/*
Constant: unknown
Logic value for unknown (maybe true xor maybe false)
*/
const unknown: t = 0u8;
/*
Constant: true
Logic value for truth
*/
const true: t = 1u8;
/*
Constant: false
Logic value for falsehood
*/
const false: t = 2u8;
/* Function: not
Negation/Inverse
*/
pure fn not(v: t) -> t { ((v << 1u8) | (v >> 1u8)) & b01 }
/* Function: and
Conjunction
*/
pure fn and(a: t, b: t) -> t { ((a | b) & b1) | ((a & b) & b0) }
/* Function: or
Disjunction
*/
pure fn or(a: t, b: t) -> t { ((a & b) & b1) | ((a | b) & b0) }
/*
Function: xor
Exclusive or
*/
pure fn xor(a: t, b: t) -> t {
let anb = a & b;
let aob = a & not(b);
ret ((anb & b1) | (anb << 1u8) | (aob >> 1u8) | (aob & b0)) & b01;
}
/*
Function: implies
Classic implication, i.e. from `a` follows `b`
*/
pure fn implies(a: t, b: t) -> t {
ret ((a & b1) >> 1u8) | (b & b0) | ((a << 1u8) & b & b1);
}
/*
Predicate: eq
Returns:
true if truth values `a` and `b` are indistinguishable in the logic
*/
pure fn eq(a: t, b: t) -> bool { a == b }
/*
Predicate: ne
Returns:
true if truth values `a` and `b` are distinguishable in the logic
*/
pure fn ne(a: t, b: t) -> bool { a != b }
/*
Predicate: is_true
Returns:
true if `v` represents truth in the logic
*/
pure fn is_true(v: t) -> bool { v == tri::true }
/*
Predicate: is_false
Returns:
true if `v` represents false in the logic
*/
pure fn is_false(v: t) -> bool { v == tri::false }
/*
Predicate: is_unknown
Returns:
true if `v` represents the unknown state in the logic
*/
pure fn is_unknown(v: t) -> bool { v == unknown }
/*
Function: from_str
Parse logic value from `s`
*/
pure fn from_str(s: str) -> t {
alt s {
"unknown" { unknown }
"true" { tri::true }
"false" { tri::false }
}
}
/*
Function: to_str
Convert `v` into a string
*/
pure fn to_str(v: t) -> str {
// FIXME replace with consts as soon as that works
alt v {
0u8 { "unknown" }
1u8 { "true" }
2u8 { "false" }
}
}
/*
Function: all_values
Iterates over all truth values by passing them to `blk`
in an unspecified order
*/
fn all_values(blk: block(v: t)) {
blk(tri::false);
blk(unknown);
blk(tri::true);
}
/*
Function: to_bit
Returns:
An u8 whose first bit is set if `if_true(v)` holds
*/
fn to_bit(v: t) -> u8 { v & b0 }
// Local Variables:
// mode: rust;
// fill-column: 78;
// indent-tabs-mode: nil
// c-basic-offset: 4
// buffer-file-coding-system: utf-8-unix
// End:
|
