Ex1/src/gate.rs

use crate::gate_type::GateType;

#[derive(Debug, Clone)]
pub struct Gate {
    pub gate_type: GateType,
    pub input_gates_indices: Vec<usize>, // Which previous gates to use
    pub input_bits_indices: Vec<usize>,  // Which input bits to use
}

impl Gate {
    pub fn new(
        gate_type: GateType,
        input_gates_indices: Vec<usize>,
        input_bits_indices: Vec<usize>,
    ) -> Self {
        let num_input_wires = input_bits_indices.len() + input_gates_indices.len();

        if num_input_wires == 0 {
            panic!("A gate without inputs is invalid");
        }

        if gate_type == GateType::Bigger && num_input_wires != 2 {
            if num_input_wires != 2 {
                panic!("Bigger gates need exactly two inputs")
            }
        }

        if gate_type == GateType::Equal && num_input_wires != 2 {
            if num_input_wires != 2 {
                panic!("Equal gates need exactly two inputs")
            }
        }
        Self {
            gate_type,
            input_gates_indices,
            input_bits_indices,
        }
    }

    pub fn eval(&self, input_bits: &Vec<bool>, evaluated_gates: &Vec<bool>) -> bool {
        /*
            The goal is to perform short-circuit evaluation with the minimum number of vector lookups.
        */

        match self.gate_type {
            /*
                A multi-input gate that outputs 1 if all input bits are 1, and 0 otherwise
            */
            GateType::And => {

                for &index in self.input_bits_indices.iter(){
                    if !input_bits[index] {
                        return false;
                    }
                }
                for &index in self.input_gates_indices.iter(){
                    if !evaluated_gates[index] {
                        return false;
                    }
                }

                return true;
            }

            /*
                A multi-input gate that outputs 1 if at least one input bit is 1, and 0 otherwise.   
            */
            GateType::Or => {
                for &index in self.input_bits_indices.iter(){
                    if input_bits[index] {
                        return true;
                    }
                }
                for &index in self.input_gates_indices.iter(){
                    if evaluated_gates[index] {
                        return true;
                    }
                }
                return false;
            },

            /*
                A gate with exactly two inputs. It outputs 1 if the two input bits are equal 
                (i.e., both 0 or both 1), and 0 otherwise
            */            
            GateType::Equal => match self.input_bits_indices.len() {
                0 => evaluated_gates[self.input_gates_indices[0]] == evaluated_gates[self.input_gates_indices[1]],
                1 => input_bits[self.input_bits_indices[0]] == evaluated_gates[self.input_gates_indices[0]],
                2 => {
                    input_bits[self.input_bits_indices[0]] == input_bits[self.input_bits_indices[1]]
                },
                _ => panic!{"How'd we get here?"}
            },

            /*
                A gate with exactly two inputs. It outputs 1 if the first input is 1 and the second
                input is 0; otherwise, it outputs 0. This gate is used to compare individual bits a_i > b_i

                A B | A > B | A && !B
                --------------------
                0 0 |   0   | 0 0  1
                0 1 |   0   | 0 0  0
                1 0 |   1   | 1 1  1
                1 1 |   0   | 1 0  0
            */
            GateType::Bigger => match self.input_bits_indices.len() {
                // bits: [] gates: [A,B]
                0 => evaluated_gates[self.input_gates_indices[0]] && !evaluated_gates[self.input_gates_indices[1]],
                
                // bits: [A] gates: [B]
                1 => input_bits[self.input_bits_indices[0]] && !evaluated_gates[self.input_gates_indices[0]],
                
                // bits: [A,B] gates: []
                2 => {
                    input_bits[self.input_bits_indices[0]] && !input_bits[self.input_bits_indices[1]]
                },

                _ => panic!{"How'd we get here? {}", input_bits.len()}
            },
        }
    }
}