usb: Make two-byte flux encoding slightly more efficient.

Use the new encoding scheme on the write path too (host -> device).

scripts/greaseweazle/usb.py

@@ -291,7 +291,7 @@ class Unit:
                         raise error.Fatal("Bad opcode in flux stream (%d)"
                                           % opcode)
                 else:
-                    val = (i - 249) * 250
+                    val = 250 + (i - 250) * 255
                     val += next(dat_i) - 1
                     flux.append(val)
                     ticks_since_index += val
@@ -311,12 +311,13 @@ class Unit:
             elif val < 250:
                 dat.append(val)
             else:
-                high = val // 250
-                if high <= 5:
-                    dat.append(249+high)
-                    dat.append(1 + val%250)
+                high = (val-250) // 255
+                if high < 5:
+                    dat.append(250 + high)
+                    dat.append(1 + (val-250) % 255)
                 else:
                     dat.append(255)
+                    dat.append(FluxOp.LongFlux)
                     dat.append(1 | (val<<1) & 255)
                     dat.append(1 | (val>>6) & 255)
                     dat.append(1 | (val>>13) & 255)

src/floppy.c

@@ -344,13 +344,13 @@ static void rdata_encode_flux(void)
             /* 1-249: One byte. */
             u_buf[U_MASK(u_prod++)] = ticks;
         } else {
-            unsigned int high = ticks / 250;
-            if (high <= 5) {
-                /* 250-1499: Two bytes. */
-                u_buf[U_MASK(u_prod++)] = 249 + high;
-                u_buf[U_MASK(u_prod++)] = 1 + (ticks % 250);
+            unsigned int high = (ticks-250) / 255;
+            if (high < 5) {
+                /* 250-1524: Two bytes. */
+                u_buf[U_MASK(u_prod++)] = 250 + high;
+                u_buf[U_MASK(u_prod++)] = 1 + ((ticks-250) % 255);
             } else {
-                /* 1500-(2^28-1): Six bytes */
+                /* 1525-(2^28-1): Six bytes */
                 u_buf[U_MASK(u_prod++)] = 0xff;
                 u_buf[U_MASK(u_prod++)] = FLUXOP_LONGFLUX;
                 u_buf[U_MASK(u_prod++)] = 1 | (ticks << 1);
@@ -521,13 +521,13 @@ static unsigned int _wdata_decode_flux(timcnt_t *tbuf, unsigned int nr)
             if ((uint32_t)(u_prod - u_cons) < 2)
                 goto out;
             u_cons++;
-            x = (x - 249) * 250;
+            x = 250 + (x - 250) * 255;
             x += u_buf[U_MASK(u_cons++)] - 1;
         } else {
-            /* 255: Five bytes */
-            if ((uint32_t)(u_prod - u_cons) < 5)
+            /* 255: Six bytes */
+            if ((uint32_t)(u_prod - u_cons) < 6)
                 goto out;
-            u_cons++;
+            u_cons += 2; /* skip 255, 1 */
             x  = (u_buf[U_MASK(u_cons++)]       ) >>  1;
             x |= (u_buf[U_MASK(u_cons++)] & 0xfe) <<  6;
             x |= (u_buf[U_MASK(u_cons++)] & 0xfe) << 13;