@@ -148,6 +148,11 @@ pub fn part2(input: &[u8]) -> usize {
148148 let upper = size * 10_000 ;
149149 assert ! ( lower <= start && start < upper) ;
150150
151+ compute ( & digits, size, start, upper)
152+ }
153+
154+ #[ cfg( not( feature = "simd" ) ) ]
155+ fn compute ( digits : & [ usize ] , size : usize , start : usize , upper : usize ) -> usize {
151156 let mut coefficients = [ 0 ; 8 ] ;
152157 let mut result = [ 0 ; 8 ] ;
153158
@@ -163,6 +168,30 @@ pub fn part2(input: &[u8]) -> usize {
163168 result. fold_decimal ( )
164169}
165170
171+ #[ cfg( feature = "simd" ) ]
172+ fn compute ( digits : & [ usize ] , size : usize , start : usize , upper : usize ) -> usize {
173+ use std:: simd:: Mask ;
174+ use std:: simd:: Simd ;
175+
176+ let mask: Mask < i32 , 8 > = Mask :: from_bitmask ( 1 ) ;
177+ let tens: Simd < u32 , 8 > = Simd :: splat ( 10 ) ;
178+
179+ let mut coefficients: Simd < u32 , 8 > = Simd :: splat ( 0 ) ;
180+ let mut result: Simd < u32 , 8 > = Simd :: splat ( 0 ) ;
181+
182+ for ( k, index) in ( start..upper) . enumerate ( ) {
183+ coefficients = mask. select (
184+ Simd :: splat ( binomial_mod_10 ( k + 99 , k) as u32 ) ,
185+ coefficients. rotate_elements_right :: < 1 > ( ) ,
186+ ) ;
187+
188+ let next = Simd :: splat ( digits[ index % size] as u32 ) ;
189+ result += next * coefficients;
190+ }
191+
192+ ( result % tens) . to_array ( ) . fold_decimal ( ) as usize
193+ }
194+
166195/// Computes C(n, k) % 2
167196///
168197/// This collapses to a special case of a product of only 4 possible values:
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