C  RELU — rectified linear unit activation, double-precision,
C  applied element-wise to a 1D vector.
C
C  Reference: V. Nair and G. Hinton, "Rectified Linear Units
C  Improve Restricted Boltzmann Machines", ICML 2010. Standard
C  default activation in most modern feed-forward networks.
C
C  Hand-written reference for the Dark Factory's Phase 3 inference
C  kernel ladder, 2026-05-24. Public domain.
C
C  Mathematical definition (ReLU activation):
C    For every index I in 1..N:
C      Y(I) = max(0, X(I))
C
C  Shapes:
C    X is a vector of length N
C    Y is a vector of length N
C
C  Note: in-place application (Y = X) is allowed at the call site;
C  the implementation reads X(I) once before writing Y(I).

      SUBROUTINE RELU (N, X, Y)
C  Inputs:
C    N    — vector length (non-negative)
C    X    — N-element input vector
C  Output:
C    Y    — N-element output vector; Y(I) = max(0, X(I)) for each I
      INTEGER N
      DOUBLE PRECISION X(*), Y(*)
      INTEGER I
      DOUBLE PRECISION V
      DO 10 I = 1, N
         V = X(I)
         IF (V .GT. 0.0D0) THEN
            Y(I) = V
         ELSE
            Y(I) = 0.0D0
         END IF
   10 CONTINUE
      RETURN
      END