Explanation of this prefix Sum Coding Challenge from Codility GenomicRangeQuery in Swift 4.2 - swift

Question

Could someone explain how this prefix sum calculation work as I am confused I mean I understand that it creates an array of Ints with the prefix-sums of the letters but I do not understand how? Could someone perhaps post a more naive logic or some explanation? Or perhaps a shorter version of that MAP function as it is all confusingly complicated.
Link to the challenge just in case someone would like to have a look at it
public func solution(_ S : inout String, _ P : inout [Int], _ Q : inout [Int]) -> [Int] {
//The mapping below needs the explanation
var prefixSumA = S.map({ () -> (Character) -> Int in
var s = 0; return {
if $0 == "A" {
return (s += 1, s).1
}
return s
}
}())//until here
prefixSumA.insert(0, at: 0)
var prefixSumC = S.map({ () -> (Character) -> Int in
var s = 0; return {
if $0 == "C" {
return (s += 1, s).1
}
return s
}
}())
prefixSumC.insert(0, at: 0)
var prefixSumG = S.map({ () -> (Character) -> Int in
var s = 0; return {
if $0 == "G" {
return (s += 1, s).1
}
return s
}
}())
prefixSumG.insert(0, at: 0)
let iterations = min(P.count, Q.count)
var result = [Int]()
for i in 0...iterations - 1 {
let p = P[i]
let q = Q[i] + 1
if prefixSumA[q] - prefixSumA[p] > 0 {
result.append(1)
} else if prefixSumC[q] - prefixSumC[p] > 0 {
result.append(2)
} else if prefixSumG[q] - prefixSumG[p] > 0 {
result.append(3)
} else {
result.append(4)
}
}
return result
}

Answer 1

prefixSumA calculates the number of As from the start to every given index. The same can be said about prefixSumC, and prefixSumG.
For example, if the string S is "CAGCCTA", we'll have:
prefixSumA = [0, 0, 1, 1, 1, 1, 1, 2]
prefixSumC = [0, 1, 1, 1, 2, 3, 3, 3]
prefixSumG = [0, 0, 0, 1, 1, 1, 1, 1]
(Notice the zero inserted at the beginning.)
This block of code:
var prefixSumA = S.map({ () -> (Character) -> Int in
var s = 0; return {
if $0 == "A" {
return (s += 1, s).1
}
return s
}
}())
prefixSumA.insert(0, at: 0)
returns a closure that, if the character is A, would capture the current value of s (which is the last value in the array), increment it, end return it (s).
return (s += 1, s).1 is a fancy way, that could be written as:
s += 1
return s
The whole block can be written more simply:
var prefixSumA = [0]
var countOfA = 0
prefixSumA += S.map { char in
if char == "A" { countOfA += 1 }
return countOfA
}
The same can be done for prefixSumC and prefixSumG.

Answer 2

I tried this one and got 62%. Performance Tests Fails.
public func solution(_ S : inout String, _ P : inout [Int], _ Q : inout [Int]) -> [Int] {
// write your code in Swift 4.2.1 (Linux)
var retArr = [Int]()
var chrArr = [Character]()
for chr in S {
chrArr.append(chr)
}
for i in 0..<P.count {
var minFactor = 4
if P[i] - Q[i] == 0 {
if chrArr[P[i]] == "A"{
minFactor = 1
}else if chrArr[P[i]] == "C"{
minFactor = 2
}else if chrArr[P[i]] == "G"{
minFactor = 3
}
}else {
for j in P[i]...Q[i] {
if chrArr[j] == "A"{
minFactor = 1
break
}else if chrArr[j] == "C"{
minFactor = 2
}else if chrArr[j] == "G"{
if minFactor > 2 {
minFactor = 3
}
}
}
}
retArr.append(minFactor)
}
return retArr
}