Merge pull request #67 from drozzy/patch-2

Update risk.py

simglucose/analysis/risk.py

@@ -1,17 +1,40 @@
 import numpy as np
-import warnings
-
 
 def risk_index(BG, horizon):
     # BG is in mg/dL
-    # horizon in samples
-    with warnings.catch_warnings():
-        warnings.simplefilter('ignore')
-        BG_to_compute = BG[-horizon:]
-        fBG = 1.509 * (np.log(BG_to_compute)**1.084 - 5.381)
-        rl = 10 * fBG[fBG < 0]**2
-        rh = 10 * fBG[fBG > 0]**2
-        LBGI = np.nan_to_num(np.mean(rl))
-        HBGI = np.nan_to_num(np.mean(rh))
-        RI = LBGI + HBGI
+    BG_to_compute = BG[-horizon:]
+    risks =[risk(r) for r in BG_to_compute]
+    LBGI = np.mean([r[0] for r in risks])
+    HBGI = np.mean([r[1] for r in risks])
+    RI = np.mean([r[2] for r in risks])
+
     return (LBGI, HBGI, RI)
+
+def risk(BG):
+    """
+    Risk is a percentage - ranging from 0 to 100%.
+    The 20 and 600 mg/dl are just the values to which the risk formula was fit. 
+    The aim is to make the risk maximum when it is either 20 or 600.
+    The units in the paper below are different (mmol/l), but in our units (mg/dl) these limits are 20 and 600.
+
+    Reference, in particular see appendix for the derivation of risk:
+    https://diabetesjournals.org/care/article/20/11/1655/21162/Symmetrization-of-the-Blood-Glucose-Measurement
+
+    """
+    MIN_BG = 20.0
+    MAX_BG = 600.0
+    if BG <= MIN_BG: 
+        return (100.0, 0.0, 100.0)
+    if BG >= MAX_BG:
+        return (0.0, 100.0, 100.0)
+
+    U = 1.509 * (np.log(BG)**1.084 - 5.381)
+
+    ri = 10 * U**2
+
+    rl, rh = 0.0, 0.0
+    if U <= 0:
+        rl = ri
+    if U >= 0:
+        rh = ri
+    return (rl, rh, ri)