Gradually and randomly visualize string - ios

Question

I am currently working on a simple program to gradually and randomly visualize a string in two iteration. Right now I have managed to get the first iteration but I'm not sure how to do the second one. If someone could give any example or advice I would be very grateful. My code looks like this:
let s = "Hello playground"
let factor = 0.25
let factor2 = 0.45
var n = s.filter({ $0 != " " }).count // # of non-space characters
var m = lrint(factor * Double(n)) // # of characters to display
let t = String(s.map { c -> Character in
if c == " " {
// Preserve space
return " "
} else if Int.random(in: 0..<n) < m {
// Replace
m -= 1
n -= 1
return c
} else {
// Keep
n -= 1
return "_"
}
})
print(t) // h_l__ _l_______d
To clarify, I want to use factor2 in the second iteration to print something that randomly add letters on top of t that looks something like this h_l_o pl_g_____d.

Answer 1

Replacing Characters
Starting from #MartinR's code, you should remember the indices that have been replaced. So, I am going to slightly change the code that replaces characters :
let s = "Hello playground"
let factor = 0.25
let factor2 = 0.45
var n = s.filter({ $0 != " " }).count // # of non-space characters
let nonSpaces = n
var m = lrint(factor * Double(n)) // # of characters to display
var indices = Array(s.indices)
var t = ""
for i in s.indices {
let c = s[i]
if c == " " {
// Preserve space
t.append(" ")
indices.removeAll(where: { $0 == i })
} else if Int.random(in: 0..<n) < m {
// Keep
m -= 1
n -= 1
t.append(c)
indices.removeAll(where: { $0 == i })
} else {
// Replace
n -= 1
t.append("_")
}
}
print(t) //For example: _e___ ______ou_d
Revealing Characters
In order to do that, we should calculate the number of characters that we want to reveal:
m = lrint((factor2 - factor) * Double(nonSpaces))
To pick three indices to reveal randomly, we shuffle indices and then replace the m first indices :
indices.shuffle()
var u = t
for i in 0..<m {
let index = indices[i]
u.replaceSubrange(index..<u.index(after: index), with: String(s[index]))
}
indices.removeSubrange(0..<m)
print(u) //For example: _e__o _l__g_ou_d

Answer 2

I wrote StringRevealer struct, that handle all revealing logic for you:
/// Hide all unicode letter characters as `_` symbol.
struct StringRevealer {
/// We need mapping between index of string character and his position in state array.
/// This struct represent one such record
private struct Symbol: Hashable {
let index: String.Index
let position: Int
}
private let originalString: String
private var currentState: [Character]
private let charactersCount: Int
private var revealed: Int
var revealedPercent: Double {
return Double(revealed) / Double(charactersCount)
}
private var unrevealedSymbols: Set<Symbol>
init(_ text: String) {
originalString = text
var state: [Character] = []
var symbols: [Symbol] = []
var count = 0
var index = originalString.startIndex
var i = 0
while index != originalString.endIndex {
let char = originalString[index]
if CharacterSet.letters.contains(char.unicodeScalars.first!) {
state.append("_")
symbols.append(Symbol(index: index, position: i))
count += 1
} else {
state.append(char)
}
index = originalString.index(after: index)
i += 1
}
currentState = state
charactersCount = count
revealed = 0
unrevealedSymbols = Set(symbols)
}
/// Current state of text. O(n) conplexity
func text() -> String {
return currentState.reduce(into: "") { $0.append($1) }
}
/// Reveal one random symbol in string
mutating func reveal() {
guard let symbol = unrevealedSymbols.randomElement() else { return }
unrevealedSymbols.remove(symbol)
currentState[symbol.position] = originalString[symbol.index]
revealed += 1
}
/// Reveal random symbols on string until `revealedPercent` > `percent`
mutating func reveal(until percent: Double) {
guard percent <= 1 else { return }
while revealedPercent < percent {
reveal()
}
}
}
var revealer = StringRevealer("Hello товарищ! 👋")
print(revealer.text())
print(revealer.revealedPercent)
for percent in [0.25, 0.45, 0.8] {
revealer.reveal(until: percent)
print(revealer.text())
print(revealer.revealedPercent)
}
It use CharacterSet.letters inside, so most of languages should be supported, emoji ignored and not-alphabetic characters as well.