Swift "can not increment endIndex" when deleting Japanese characters - ios

Question

I'm using Swift 2.0/Xcode 7.0 for an iOS app. I've built a Japanese IME to convert Roman characters to their Japanese equivalent. For example:
Typing "a" will convert the "a" to "あ"
Typing a "k" will do nothing (no match in Japanese) until the next character is typed, for example typing a "k" then an "a" will result in "か"
My problem is when I try to delete a Japanese character. If there is only one character in the text field, the delete key/function works as expected. However, if there is more than one, when I try to delete the character, I receive a can not increment endIndex error in the following code.
var imeInputLength: Int = 0
let currentInputValue: String = txtfldYourResponse.text!.lowercaseString
if(currentInputValue.characters.count==0) {
imeInputLength = 0
}
let inputStringToKeep: String = currentInputValue.substringWithRange(
Range<String.Index>(start: currentInputValue.startIndex.advancedBy(imeInputLength),
end: currentInputValue.endIndex))
let imeStringToKeep: String = currentInputValue.substringWithRange(
Range<String.Index>(start: currentInputValue.startIndex,
end: currentInputValue.startIndex.advancedBy(imeInputLength)))
if let imeValueDC = JIMEDC[inputStringToKeep] {
txtfldYourResponse.text = imeStringToKeep + imeValueDC
imeInputLength = (txtfldYourResponse.text?.characters.count)!-1
}
if let imeValue = JIME[inputStringToKeep] {
txtfldYourResponse.text = imeStringToKeep + imeValue
imeInputLength = txtfldYourResponse.text!.characters.count
}
currentInputValue is the text from the text field.
imeInputLength is an int (initial value = 0) that increments by the total character count in the text field AFTER a match is found.
JIMEDC and JIME are key/value pairs that handle the conversion from Roman to Japanese characters.
I'm printing the endIndex to the console before the code runs. It appears to increment/decrement as expected, but the code block above fails with the increment error.
I've been banging my head against this for a couple weeks now with no progress.
Edit: Clarified handling of imeInputLength and added additional code for clarification.

Answer 1

A side conversation with l'L'l got me on the right track. I was confused by the error message can not increment endIndex. In reality, I was creating an out-of-bounds error on my range.
When the backspace key was pressed, I wasn't modifying imeInputLength to reflect the new length of the input string. As a result, my advanceByamount was larger than the updated endIndex value. Swift threw this as can not increment endIndex but the reason for it was the startIndex was now greater than the endIndex.
I resolved this by first checking for the following:
if(imeInputLength > currentInputValue.characters.count) {
print("You must have pressed backspace")
imeInputLength = txtfldYourResponse.text!.characters.count
} else {
let inputStringToKeep:
...
}
If the backspace key IS pressed, I needed to see if the imeInputLength value was greater than the current character count of the input field. The only use case where imeInputLength ends up greater is when a Japanese character is deleted using the backspace key. When this condition is caught, I reset imeInputLength to the current character count and exit the routine.
I'm happy to say that all of my use cases now test out positive!