Programming of built in timer of arduino - timer

Question

I am new to programming in IDE. I am using built-in timer of arduino.I am using TDR method to capture reflection from fault location in cable and for that I am using timer.Timer should start as soon as output is send and stop at reflection. I have a reference code for it but I dont able to understand it, so if anyone know about it , it would be great.
void setup()
{
pinMode(stepPin, OUTPUT);
pinMode(refPin, OUTPUT);
pinMode(shutdownPin, OUTPUT);
TCCR1A = 0;
TCCR1B = (1 << ICNC1); // input capture noise canceller enabled, capture on falling edge
TIMSK1 = 0; // timer 1 interrupts disabled
ACSR = 0; // input capture from ICP1 pin
TCCR2B = (1 << CS20); // change timer 2 PWM frequency to 31.25kHz because we're using pin 11 as a DAC
Serial.begin(19200);
}
struct Step
{
unsigned int time;
unsigned int amplitude;
};
// Take a single measurement, using either a positive or negative edge from the comparator.
// The comparator reference voltage must have been set up and allowed to stablise before calling this.
unsigned int takeMeasurement(bool posEdge)
{
byte reg1b = (posEdge) ? 0 : (1 << ICES1); // input capture noise canceller csdisabled, set up input capture polarity, stop timer
reg1b |= (1 << CS10);
TCCR1B = reg1b;
TCNT1H = 0;
TCNT1L = 0; // clear timer
unsigned int capture = 0;
unsigned long start = micros(); // get the time
cli();
TCNT1H = 0;
TCNT1L = 0; // clear timer
TIFR1 = (1 << ICF1); // clear timer 1 input capture bit
PORTD |= (1 << 4); // set output high
sei();
do
{
if ((TIFR1 & (1 << ICF1)) && capture == 0)
{
byte temp = ICR1L;
capture = (ICR1H << 8) | temp;
}
} while (micros() - start < 100);
PORTD &= ~(1 << 4); // set output low
return capture;
}
size_t findSteps(bool positive, struct Step *results, size_t maxResults)
{
byte amplitude = (positive) ? 5 : 250;
analogWrite(refPin, amplitude);
delay(100); // wait 100ms for the output to stabilise
unsigned int lastReading = 0;
size_t numResults = 0;
unsigned int stepSize = 0; // 0 means not in a step
#ifdef DEBUG
Serial.print((positive) ? "pos " : "neg ");
#endif
for (int i = 0; i < 50; ++i)
{
analogWrite(refPin, amplitude);
delay(10);
unsigned int currentReading = takeMeasurement(positive);
unsigned int currentDiff = currentReading - lastReading; // diff since start of possible step
if (stepSize == 0)
{
// Not currently in a step
if (i != 0 && currentReading != 0 && currentDiff == 0)
{
// Found the start of a possible step
++stepSize;
}
lastReading = currentReading;
}
else
{
if (currentDiff > 2 || i + 1 == 50)
{
// Step has endeed, so record it if it is big enough
if (stepSize >= 2)
{
results->time = lastReading;
results->amplitude = amplitude - 5;
++results;
++numResults;
if (numResults == maxResults) break;
}
stepSize = 0;
lastReading = currentReading;
}
else if (currentDiff == 0)
{
++stepSize;
}
}
#ifdef DEBUG
if (i != 0) Serial.write(',');
Serial.print(currentReading);
#endif
if (positive)
{
amplitude += 5;
}
else
{
amplitude -= 5;
}
}
#ifdef DEBUG
Serial.println();
#endif
return numResults;
}