audit: additional verifications (#78)

Audit: adding more verifications to avoid undefined operations

src/operations/time_series/convert.cairo

@@ -17,7 +17,7 @@ fn div_decimals(a_price: u128, b_price: u128, output_decimals: u128) -> u128 {
 
     assert(power <= MAX_POWER, 'Conversion overflow');
     assert(a_price <= MAX_POWER, 'Conversion overflow');
-
+    assert(b_price > 0, 'Division by zero');
     a_price * power / b_price
 }
 

src/operations/time_series/metrics.cairo

@@ -130,8 +130,10 @@ fn _sum_volatility(arr: Span<TickElem>) -> Fixed {
         let prev_val = *arr.at(cur_idx - 1);
         let cur_value = cur_val.value;
         let prev_value = prev_val.value;
+        assert(prev_value.mag > 0, 'failed to compute vol');
         let cur_timestamp = cur_val.tick;
         let prev_timestamp = prev_val.tick;
+        assert(cur_timestamp > prev_timestamp, 'failed to compute vol');
         if (prev_timestamp > cur_timestamp) {
             //edge case
             assert(1 == 1, 'failed to compute vol');

src/operations/time_series/scaler.cairo

@@ -10,6 +10,7 @@ use debug::PrintTrait;
 use cubit::f128::types::fixed::{FixedTrait, Fixed, FixedPrint, ONE_u128};
 
 fn calculate_slope(x1: Fixed, x2: Fixed, y1: Fixed, y2: Fixed) -> Fixed {
+    assert(x2 > x1, 'cannot compute slope(x2<x1)');
     (y2 - y1) / (x2 - x1)
 }
 
@@ -18,6 +19,8 @@ fn calculate_slope(x1: Fixed, x2: Fixed, y1: Fixed, y2: Fixed) -> Fixed {
 fn scale_data(
     start_tick: u64, end_tick: u64, tick_array: Span<TickElem>, num_intervals: u32
 ) -> Array<TickElem> {
+    assert(num_intervals > 1, 'intervals must be > than 1');
+    assert(start_tick <= end_tick, 'start_tick must be < end_tick');
     let interval = (end_tick - start_tick) / (num_intervals.into() - 1);
 
     let mut output: Array<TickElem> = ArrayTrait::new();