Optimize ADC temperature search

Use a binary search to find our target temperate in fewer
comparisons.  The search algorithm is ostensibly slower because
it involves a division, but it's a div-by-two so should be
optimized into a simple bit-shift.  Fewer comparisons involves
fewer pgm_read_words and should be faster overall, but the gain
is arguably tiny.

Note by Traumflug:

According to @Wurstnase's performance measurements, gain is
actually pretty huge. Temp conversions 0..1024 before this
and the previous few commits:

  minimum: 1011.84 clock cycles.
  maximum: 1993.92 clock cycles.
  average: 1768.85 clock cycles.

Now:

  minimum: 437.72 clock cycles.
  maximum: 494.76 clock cycles.
  average: 470.278 clock cycles.

That's a speedup by factor 3.7 on average!

temp.c

@@ -186,72 +186,72 @@ static uint16_t mcp3008_read(uint8_t channel) {
 */
 #if defined TEMP_THERMISTOR || defined TEMP_MCP3008
 static uint16_t temp_table_lookup(uint16_t temp, uint8_t sensor) {
-  uint8_t j;
+  uint8_t lo, hi;
   uint8_t table_num = temp_sensors[sensor].additional;
 
-  for (j = 1; j < NUMTEMPS; j++) {
+  // Binary search for table value bigger than our target.
-    if (pgm_read_word(&(temptable[table_num][j][0])) > temp) {
+  //
-
+  //   lo = index of highest entry less than target.
-      if (DEBUG_PID && (debug_flags & DEBUG_PID))
+  //   hi = index of lowest entry greater than or equal to target.
-        sersendf_P(PSTR("pin:%d Raw ADC:%d table entry: %d"),
+  for (lo = 0, hi = NUMTEMPS - 1; hi - lo > 1; ) {
-                   temp_sensors[sensor].temp_pin, temp, j);
+    uint8_t j = lo + (hi - lo) / 2 ;
-
+    if (pgm_read_word(&(temptable[table_num][j][0])) >= temp)
-      if (sizeof(temptable[0][0]) == 2 * sizeof(uint16_t)) {
+      hi = j ;
-        /**
+    else
-          This code handles temptables with value pairs and is deprecated.
+      lo = j ;
-          It's kept for compatibility with legacy, handcrafted tables, only.
-
-          The new code expects tables with triples, nevertheless it's smaller
-          and also faster. Configtool was already changed to create tables
-          with triples, only.
-        */
-        // Wikipedia's example linear interpolation formula.
-        // y = ((x - x₀)y₁ + (x₁-x)y₀) / (x₁ - x₀)
-        // y = temp
-        // x = ADC reading
-        // x₀= temptable[j-1][0]
-        // x₁= temptable[j][0]
-        // y₀= temptable[j-1][1]
-        // y₁= temptable[j][1]
-        temp = (
-        // ((x - x₀)y₁
-          ((uint32_t)temp - pgm_read_word(&(temptable[table_num][j-1][0]))) *
-                            pgm_read_word(&(temptable[table_num][j][1]))
-        //             +
-          +
-        //               (x₁-x)y₀)
-          (pgm_read_word(&(temptable[table_num][j][0])) - (uint32_t)temp) *
-            pgm_read_word(&(temptable[table_num][j - 1][1])))
-        //                        /
-          /
-        //                          (x₁ - x₀)
-          (pgm_read_word(&(temptable[table_num][j][0])) -
-           pgm_read_word(&(temptable[table_num][j - 1][0])));
-      } else
-      if (sizeof(temptable[0][0]) == 3 * sizeof(uint16_t)) {
-        // Linear interpolation using pre-computed slope.
-        // y = y₁ - (x - x₁) * d₁
-        #define X1 pgm_read_word(&(temptable[table_num][j][0]))
-        #define Y1 pgm_read_word(&(temptable[table_num][j][1]))
-        #define D1 pgm_read_word(&(temptable[table_num][j][2]))
-
-        temp = Y1 - ((((int32_t)temp - X1) * D1 + (1 << 7)) >> 8);
-      }
-
-      if (DEBUG_PID && (debug_flags & DEBUG_PID))
-        sersendf_P(PSTR(" temp:%d.%d"), temp / 4, (temp % 4) * 25);
-
-      // Value found, no need to read the table further.
-      break;
-    }
   }
 
   if (DEBUG_PID && (debug_flags & DEBUG_PID))
-    sersendf_P(PSTR(" Sensor:%d\n"), sensor);
+    sersendf_P(PSTR("pin:%d Raw ADC:%d table entry: %d"),
+               temp_sensors[sensor].temp_pin, temp, hi);
 
-  // Clamp for overflows.
+  if (sizeof(temptable[0][0]) == 2 * sizeof(uint16_t)) {
-  if (j == NUMTEMPS)
+    /**
-    temp = temptable[table_num][NUMTEMPS - 1][1];
+      This code handles temptables with value pairs and is deprecated.
+      It's kept for compatibility with legacy, handcrafted tables, only.
+
+      The new code expects tables with triples, nevertheless it's smaller
+      and also faster. Configtool was already changed to create tables
+      with triples, only.
+    */
+    // Wikipedia's example linear interpolation formula.
+    // y = ((x - x₀)y₁ + (x₁-x)y₀) / (x₁ - x₀)
+    // y = temp
+    // x = ADC reading
+    // x₀= temptable[lo][0]
+    // x₁= temptable[hi][0]
+    // y₀= temptable[lo][1]
+    // y₁= temptable[hi][1]
+    temp = (
+      // ((x - x₀)y₁
+      ((uint32_t)temp - pgm_read_word(&(temptable[table_num][lo][0]))) *
+                        pgm_read_word(&(temptable[table_num][hi][1]))
+      //             +
+      +
+      //               (x₁-x)y₀)
+      (pgm_read_word(&(temptable[table_num][hi][0])) - (uint32_t)temp) *
+        pgm_read_word(&(temptable[table_num][lo][1])))
+      //                        /
+      /
+      //                          (x₁ - x₀)
+      (pgm_read_word(&(temptable[table_num][hi][0])) -
+       pgm_read_word(&(temptable[table_num][lo][0])));
+  } else
+  if (sizeof(temptable[0][0]) == 3 * sizeof(uint16_t)) {
+    // Linear interpolation using pre-computed slope.
+    // y = y₁ - (x - x₁) * d₁
+    #define X1 pgm_read_word(&(temptable[table_num][hi][0]))
+    #define Y1 pgm_read_word(&(temptable[table_num][hi][1]))
+    #define D1 pgm_read_word(&(temptable[table_num][hi][2]))
+
+    temp = Y1 - ((((int32_t)temp - X1) * D1 + (1 << 7)) >> 8);
+  }
+
+  if (DEBUG_PID && (debug_flags & DEBUG_PID))
+    sersendf_P(PSTR(" temp:%d.%d"), temp / 4, (temp % 4) * 25);
+
+  if (DEBUG_PID && (debug_flags & DEBUG_PID))
+    sersendf_P(PSTR(" Sensor:%d\n"), sensor);
 
   return temp;
 }