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Task/Fast-Fourier-transform/ERRE/fast-fourier-transform.erre

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+PROGRAM FFT
+
+CONST CNT=8
+
+!$DYNAMIC
+DIM REL[0],IMG[0],CMP[0],V[0]
+
+BEGIN
+   SIG=INT(LOG(CNT)/LOG(2)+0.9999)
+   REAL1=2^SIG
+
+   REAL=REAL1-1
+   REAL2=INT(REAL1/2)
+   REAL4=INT(REAL1/4)
+   REAL3=REAL4+REAL2
+
+!$DIM REL[REAL1],IMG[REAL1],CMP[REAL3]
+
+FOR I=0 TO CNT-1 DO
+   READ(REL[I],IMG[I])
+END FOR
+
+DATA(1,0,1,0,1,0,1,0,0,0,0,0,0,0,0,0)
+
+SIG2=INT(SIG/2)
+SIG1=SIG-SIG2
+CNT1=2^SIG1
+CNT2=2^SIG2
+
+!$DIM V[CNT1-1]
+V[0]=0
+DV=1
+PTR=CNT1
+
+FOR J=1 TO SIG1 DO
+  HLFPTR=INT(PTR/2)
+  PT=CNT1-HLFPTR
+  FOR I=HLFPTR TO PT STEP PTR DO
+    V[I]=V[I-HLFPTR]+DV
+  END FOR
+  DV=2*DV
+  PTR=HLFPTR
+END FOR
+
+K=2*π/REAL1
+
+FOR X=0 TO REAL4 DO
+   CMP[X]=COS(K*X)
+   CMP[REAL2-X]=-CMP[X]
+   CMP[REAL2+X]=-CMP[X]
+END FOR
+
+PRINT("FFT: BIT REVERSAL")
+
+FOR I=0 TO CNT1-1 DO
+  IP=I*CNT2
+  FOR J=0 TO CNT2-1 DO
+    H=IP+J
+    G=V[J]*CNT2+V[I]
+    IF G>H THEN
+       SWAP(REL[G],REL[H])
+       SWAP(IMG[G],IMG[H])
+    END IF
+  END FOR
+END FOR
+
+T=1
+FOR STAGE=1 TO SIG DO
+  PRINT("STAGE:";STAGE)
+  D=INT(REAL2/T)
+  FOR II=0 TO T-1 DO
+     L=D*II
+     LS=L+REAL4
+     FOR I=0 TO D-1 DO
+       A=2*I*T+II
+       B=A+T
+       F1=REL[A]
+       F2=IMG[A]
+       CNT1=CMP[L]*REL[B]
+       CNT2=CMP[LS]*IMG[B]
+       CNT3=CMP[LS]*REL[B]
+       CNT4=CMP[L]*IMG[B]
+       REL[A]=F1+CNT1-CNT2
+       IMG[A]=F2+CNT3+CNT4
+       REL[B]=F1-CNT1+CNT2
+       IMG[B]=F2-CNT3-CNT4
+     END FOR
+  END FOR
+  T=2*T
+END FOR
+
+PRINT("NUM REAL     IMAG")
+FOR I=0 TO REAL DO
+    IF ABS(REL[I])<1E-5 THEN REL[I]=0 END IF
+    IF ABS(IMG[I])<1E-5 THEN IMG[I]=0 END IF
+    PRINT(I;"";)
+    WRITE("##.###### ##.######";REL[I];IMG[I])
+END FOR
+END PROGRAM