Solving the Simple Text Editor challenge as fast as possible. See challenge here: https://www.hackerrank.com/challenges/simple-text-editor/problem

simple-text-editor.py

#!/usr/bin/env python3

# Challenge: https://www.hackerrank.com/challenges/simple-text-editor/problem
# Trying to solve it as fast as possible!

import sys
from collections import namedtuple
import re

# Inizializing global variables (I don't like global vars, it's faster like this...)
output = False  # If False, output of print operations is disabled
SHistory = []
S = ""
WTotLen = 0
kSum = 0

### Functions to manage global vars ###


def resetGlobalVars():
    global SHistory
    global S
    global WTotLen
    global kSum
    SHistory = []
    S = ""
    WTotLen = 0
    kSum = 0


def toggleOutput():
    global output
    output = not(output)


### Functions to check constraints ###
def checkIndex(k):
    # Checking constraint 2
    global S
    if (k < 1 or k > len(S)):
        raise IndexError("Index parameter k must be so that: 1 <= k <= len(S)")


def checkWTotLen(W):
    # Checking constraint 3
    global WTotLen
    WTotLen = WTotLen + len(W)
    if (WTotLen > pow(10, 6)):
        raise ValueError(
            "The sum of the lengths of all W in the input must be <= 10ˆ6")


### Functions to execute operations ###
def appendString(W):
    global S
    checkWTotLen(W)
    SHistory.append(S)
    S = S + W


def deleteLastChars(k):
    global S
    global kSum

    kSum = kSum + k

    # Checking constraint 4
    if (kSum > 2*pow(10, 6)):
        raise ValueError(
            "The sum of of all k over all delete operations must be <= 2*10ˆ6"
        )

    checkIndex(k)

    SHistory.append(S)
    S = S[0:len(S)-k]


def printChar(k):
    global S
    checkIndex(k)
    if output:
        print(S[k - 1])


def undo():
    global S
    S = SHistory.pop()


Operation = namedtuple('Operation', 'function args originalLine')
operations = []


### Main execution ###
input = sys.stdin.readlines()

# Checking constraint 1
opsCount = int(input[0])
if (opsCount < 1 or opsCount > pow(10, 6)):
    sys.exit("Unsupported numer of operations (Q): 1 <= Q <= 10ˆ6")

for line in input[1:opsCount+1]:
    if (line[-1] != "\n"):
        line = line + '\n'

    def extractParam():
        return line[2:].replace("\n", "")

    def unsupportedOperation():
        print("Unsupported operation: %s" % line)

    if (line[0] == "1"):
        # Checking constraint 5
        if not re.match(r"^1 ([a-z])+\n$", line):
            unsupportedOperation()
            sys.exit(
                "Append operation usage: 1 string\nExample: 1 asd\nAll input characters must be lowercase English letters."
            )
        operations.append(
            Operation(appendString, [extractParam()], line))
    elif(line[0] == "2"):
        if not re.match(r"^2 (\d)+\n$", line):
            unsupportedOperation()
            sys.exit("Delete operation usage: 2 number\nExample: 2 2")
        operations.append(
            Operation(deleteLastChars, [int(extractParam())], line))
    elif(line[0] == "3"):
        if not re.match(r"^3 (\d)+\n$", line):
            unsupportedOperation()
            sys.exit("Print operation usage: 3 number\nExample: 3 2")
        operations.append(Operation(printChar, [int(extractParam())], line))
    elif(line[0] == "4"):
        if line != "4\n":
            unsupportedOperation()
            sys.exit("Undo operation usage: 4\nNo parameters, no trailing space.")
        operations.append(Operation(undo, [], line))
    else:
        unsupportedOperation()
        sys.exit()


# Checking constraint 6: executing operations and checking if there are exceptions
for idx, operation in enumerate(operations):
    try:
        operation.function(*operation.args)
    except:
        print(
            "The sequence of operations given as input cannot be performed."
        )
        print(
            "The operation that triggered the error is: %s (Operation number %d)"
            % (operation.originalLine, idx)
        )
        e = sys.exc_info()
        sys.exit("Error: %s" % str(e))


# Resetting everything, enabling input, executing operations again...
# For sure there is a better method to do it :)
toggleOutput()
resetGlobalVars()

for operation in operations:
    operation.function(*operation.args)