# Documentation

Mathlib.Data.Num.Bitwise

# Bitwise operations using binary representation of integers #

## Definitions #

• bitwise operations for PosNum and Num,
• SNum, a type that represents integers as a bit string with a sign bit at the end,
• arithmetic operations for SNum.

Bitwise "or" for PosNum.

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Bitwise "and" for PosNum.

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Bitwise fun a b ↦ a && !b↦ a && !b for PosNum. For example, ldiff 5 9 = 4:

 101
1001
----
100

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Bitwise "xor" for PosNum.

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a.testBit n is true iff the n-th bit (starting from the LSB) in the binary representation of a is active. If the size of a is less than n, this evaluates to false.

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def PosNum.oneBits :
PosNum

n.oneBits 0 is the list of indices of active bits in the binary representation of n.

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Left-shift the binary representation of a PosNum.

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Right-shift the binary representation of a PosNum.

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def Num.lor :

Bitwise "or" for Num.

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def Num.land :

Bitwise "and" for Num.

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def Num.ldiff :

Bitwise fun a b ↦ a && !b↦ a && !b for Num. For example, ldiff 5 9 = 4:

 101
1001
----
100

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• = match x, x with | Num.zero, x => 0 | p, Num.zero => p | , =>
def Num.lxor :

Bitwise "xor" for Num.

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def Num.shiftl :
NumNum

Left-shift the binary representation of a Num.

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def Num.shiftr :
NumNum

Right-shift the binary representation of a Num.

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• = match x, x with | Num.zero, x => 0 | , n =>
def Num.testBit :
Num

a.testBit n is true iff the n-th bit (starting from the LSB) in the binary representation of a is active. If the size of a is less than n, this evaluates to false.

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def Num.oneBits :
Num

n.oneBits is the list of indices of active bits in the binary representation of n.

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• = match x with | Num.zero => [] | =>
inductive NzsNum :
• msb:
• Add a bit at the end of a NzsNum.

bit: Bool

This is a nonzero (and "non minus one") version of SNum. See the documentation of SNum for more details.

Instances For
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inductive SNum :
• zero:
• nz:

Alternative representation of integers using a sign bit at the end. The convention on sign here is to have the argument to msb denote the sign of the MSB itself, with all higher bits set to the negation of this sign. The result is interpreted in two's complement.

13 = ..0001101(base 2) = nz (bit1 (bit0 (bit1 (msb true)))) -13 = ..1110011(base 2) = nz (bit1 (bit1 (bit0 (msb false))))

As with Num, a special case must be added for zero, which has no msb, but by two's complement symmetry there is a second special case for -1. Here the Bool field indicates the sign of the number.

0 = ..0000000(base 2) = zero false -1 = ..1111111(base 2) = zero true

Instances For
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instance instZeroSNum :
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instance instOneNzsNum :
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instance instOneSNum :
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The SNum representation uses a bit string, essentially a list of 0 (false) and 1 (true) bits, and the negation of the MSB is sign-extended to all higher bits.

Add a bit at the end of a NzsNum.

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Sign of a NzsNum.

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@[match_pattern]

Bitwise not for NzsNum.

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Bitwise not for NzsNum.

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Add an inactive bit at the end of a NzsNum. This mimics PosNum.bit0.

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Add an active bit at the end of a NzsNum. This mimics PosNum.bit1.

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The head of a NzsNum is the boolean value of its LSB.

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• = match x with | => b | => b

The tail of a NzsNum is the SNum obtained by removing the LSB. Edge cases: tail 1 = 0 and tail (-2) = -1.

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def SNum.sign :

Sign of a SNum.

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• = match x with | => z | =>
@[match_pattern]
def SNum.not :

Bitwise not for SNum.

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Bitwise not for SNum.

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@[match_pattern]
def SNum.bit :
Bool

Add a bit at the end of a SNum. This mimics NzsNum.bit.

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Add a bit at the end of a SNum. This mimics NzsNum.bit.

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def SNum.bit0 :

Add an inactive bit at the end of a SNum. This mimics ZNum.bit0.

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def SNum.bit1 :

Add an active bit at the end of a SNum. This mimics ZNum.bit1.

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theorem SNum.bit_zero (b : Bool) :
SNum.bit b () =
def NzsNum.drec' {C : SNumSort u_1} (z : (b : Bool) → C ()) (s : (b : Bool) → (p : SNum) → C pC (SNum.bit b p)) (p : NzsNum) :
C ()

A dependent induction principle for NzsNum, with base cases 0 : SNum and (-1) : SNum.

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The head of a SNum is the boolean value of its LSB.

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• = match x with | => z | =>
def SNum.tail :

The tail of a SNum is obtained by removing the LSB. Edge cases: tail 1 = 0, tail (-2) = -1, tail 0 = 0 and tail (-1) = -1.

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• = match x with | => | =>
def SNum.drec' {C : SNumSort u_1} (z : (b : Bool) → C ()) (s : (b : Bool) → (p : SNum) → C pC (SNum.bit b p)) (p : SNum) :
C p

A dependent induction principle for SNum which avoids relying on NzsNum.

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def SNum.rec' {α : Sort u_1} (z : Boolα) (s : BoolSNumαα) :
SNumα

An induction principle for SNum which avoids relying on NzsNum.

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def SNum.testBit :

SNum.testBit n a is true iff the n-th bit (starting from the LSB) of a is active. If the size of a is less than n, this evaluates to false.

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def SNum.succ :

The successor of a SNum (i.e. the operation adding one).

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def SNum.pred :

The predecessor of a SNum (i.e. the operation of removing one).

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def SNum.neg (n : SNum) :

The opposite of a SNum.

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BoolBool

SNum.czAdd a b n is n + a - b (where a and b should be read as either 0 or 1). This is useful to implement the carry system in cAdd.

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def SNum.bits :
SNum(n : ) →

a.bits n is the vector of the n first bits of a (starting from the LSB).

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SNumSNum

SNum.cAdd n m a is n + m + a (where a should be read as either 0 or 1). a represents a carry bit.

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• One or more equations did not get rendered due to their size.
def SNum.add (a : SNum) (b : SNum) :

Add two SNums.

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def SNum.sub (a : SNum) (b : SNum) :

Subtract two SNums.

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def SNum.mul (a : SNum) :

Multiply two SNums.

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• = SNum.rec' (fun b => bif b then -a else 0) fun b x IH => bif b then + a else