From b8345731a477633bc82216dbc398faecafaf894f Mon Sep 17 00:00:00 2001
From: xuri <xuri.me@gmail.com>
Date: Sat, 2 Apr 2022 00:04:21 +0800
Subject: [PATCH] ref #65, new formula functions: T.DIST and TDIST

---
 calc.go      | 90 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 calc_test.go | 23 ++++++++++++++
 2 files changed, 113 insertions(+)

diff --git a/calc.go b/calc.go
index 5bad72a..6252363 100644
--- a/calc.go
+++ b/calc.go
@@ -657,6 +657,8 @@ type formulaFuncs struct {
 //    TBILLEQ
 //    TBILLPRICE
 //    TBILLYIELD
+//    T.DIST
+//    TDIST
 //    TEXTJOIN
 //    TIME
 //    TIMEVALUE
@@ -9008,6 +9010,94 @@ func (fn *formulaFuncs) STDEVdotP(argsList *list.List) formulaArg {
 	return fn.stdevp("STDEV.P", argsList)
 }
 
+// getTDist is an implementation for the beta distribution probability density
+// function.
+func getTDist(T, fDF, nType float64) float64 {
+	var res float64
+	switch nType {
+	case 1:
+		res = 0.5 * getBetaDist(fDF/(fDF+T*T), fDF/2, 0.5)
+		break
+	case 2:
+		res = getBetaDist(fDF/(fDF+T*T), fDF/2, 0.5)
+		break
+	case 3:
+		res = math.Pow(1+(T*T/fDF), -(fDF+1)/2) / (math.Sqrt(fDF) * getBeta(0.5, fDF/2.0))
+		break
+	case 4:
+		X := fDF / (T*T + fDF)
+		R := 0.5 * getBetaDist(X, 0.5*fDF, 0.5)
+		res = 1 - R
+		if T < 0 {
+			res = R
+		}
+		break
+	}
+	return res
+}
+
+// TdotDIST function calculates the one-tailed Student's T Distribution, which
+// is a continuous probability distribution that is frequently used for
+// testing hypotheses on small sample data sets. The syntax of the function
+// is:
+//
+//    T.DIST(x,degrees_freedom,cumulative)
+//
+func (fn *formulaFuncs) TdotDIST(argsList *list.List) formulaArg {
+	if argsList.Len() != 3 {
+		return newErrorFormulaArg(formulaErrorVALUE, "T.DIST requires 3 arguments")
+	}
+	var x, degrees, cumulative formulaArg
+	if x = argsList.Front().Value.(formulaArg).ToNumber(); x.Type != ArgNumber {
+		return x
+	}
+	if degrees = argsList.Front().Next().Value.(formulaArg).ToNumber(); degrees.Type != ArgNumber {
+		return degrees
+	}
+	if cumulative = argsList.Back().Value.(formulaArg).ToBool(); cumulative.Type != ArgNumber {
+		return cumulative
+	}
+	if cumulative.Number == 1 && degrees.Number < 1 {
+		return newErrorFormulaArg(formulaErrorNUM, formulaErrorNUM)
+	}
+	if cumulative.Number == 0 {
+		if degrees.Number < 0 {
+			return newErrorFormulaArg(formulaErrorNUM, formulaErrorNUM)
+		}
+		if degrees.Number == 0 {
+			return newErrorFormulaArg(formulaErrorDIV, formulaErrorDIV)
+		}
+		return newNumberFormulaArg(getTDist(x.Number, degrees.Number, 3))
+	}
+	return newNumberFormulaArg(getTDist(x.Number, degrees.Number, 4))
+}
+
+// TDIST function calculates the Student's T Distribution, which is a
+// continuous probability distribution that is frequently used for testing
+// hypotheses on small sample data sets. The syntax of the function is:
+//
+//    TDIST(x,degrees_freedom,tails)
+//
+func (fn *formulaFuncs) TDIST(argsList *list.List) formulaArg {
+	if argsList.Len() != 3 {
+		return newErrorFormulaArg(formulaErrorVALUE, "TDIST requires 3 arguments")
+	}
+	var x, degrees, tails formulaArg
+	if x = argsList.Front().Value.(formulaArg).ToNumber(); x.Type != ArgNumber {
+		return x
+	}
+	if degrees = argsList.Front().Next().Value.(formulaArg).ToNumber(); degrees.Type != ArgNumber {
+		return degrees
+	}
+	if tails = argsList.Back().Value.(formulaArg).ToNumber(); tails.Type != ArgNumber {
+		return tails
+	}
+	if x.Number < 0 || degrees.Number < 1 || (tails.Number != 1 && tails.Number != 2) {
+		return newErrorFormulaArg(formulaErrorNUM, formulaErrorNUM)
+	}
+	return newNumberFormulaArg(getTDist(x.Number, degrees.Number, tails.Number))
+}
+
 // TRIMMEAN function calculates the trimmed mean (or truncated mean) of a
 // supplied set of values. The syntax of the function is:
 //
diff --git a/calc_test.go b/calc_test.go
index 2b76ed3..321934f 100644
--- a/calc_test.go
+++ b/calc_test.go
@@ -1155,6 +1155,13 @@ func TestCalcCellValue(t *testing.T) {
 		"=STDEVP(A1:B2,6,-1)": "2.40947204913349",
 		// STDEV.P
 		"=STDEV.P(A1:B2,6,-1)": "2.40947204913349",
+		// T.DIST
+		"=T.DIST(1,10,TRUE)":   "0.82955343384897",
+		"=T.DIST(-1,10,TRUE)":  "0.17044656615103",
+		"=T.DIST(-1,10,FALSE)": "0.230361989229139",
+		// TDIST
+		"=TDIST(1,10,1)": "0.17044656615103",
+		"=TDIST(1,10,2)": "0.34089313230206",
 		// TRIMMEAN
 		"=TRIMMEAN(A1:B4,10%)": "2.5",
 		"=TRIMMEAN(A1:B4,70%)": "2.5",
@@ -3009,6 +3016,22 @@ func TestCalcCellValue(t *testing.T) {
 		// STDEV.P
 		"=STDEV.P()":     "STDEV.P requires at least 1 argument",
 		"=STDEV.P(\"\")": "#DIV/0!",
+		// T.DIST
+		"=T.DIST()":             "T.DIST requires 3 arguments",
+		"=T.DIST(\"\",10,TRUE)": "strconv.ParseFloat: parsing \"\": invalid syntax",
+		"=T.DIST(1,\"\",TRUE)":  "strconv.ParseFloat: parsing \"\": invalid syntax",
+		"=T.DIST(1,10,\"\")":    "strconv.ParseBool: parsing \"\": invalid syntax",
+		"=T.DIST(1,0,TRUE)":     "#NUM!",
+		"=T.DIST(1,-1,FALSE)":   "#NUM!",
+		"=T.DIST(1,0,FALSE)":    "#DIV/0!",
+		// TDIST
+		"=TDIST()":          "TDIST requires 3 arguments",
+		"=TDIST(\"\",10,1)": "strconv.ParseFloat: parsing \"\": invalid syntax",
+		"=TDIST(1,\"\",1)":  "strconv.ParseFloat: parsing \"\": invalid syntax",
+		"=TDIST(1,10,\"\")": "strconv.ParseFloat: parsing \"\": invalid syntax",
+		"=TDIST(-1,10,1)":   "#NUM!",
+		"=TDIST(1,0,1)":     "#NUM!",
+		"=TDIST(1,10,0)":    "#NUM!",
 		// TRIMMEAN
 		"=TRIMMEAN()":        "TRIMMEAN requires 2 arguments",
 		"=TRIMMEAN(A1,\"\")": "strconv.ParseFloat: parsing \"\": invalid syntax",