Problem Description

This problem asks you to create a generator function that produces a sequence of dates within a given range.

You're given three parameters:

• start: A string representing the starting date in format "YYYY-MM-DD"
• end: A string representing the ending date in format "YYYY-MM-DD"
• step: A positive integer indicating how many days to skip between each generated date

The generator should yield dates starting from start and continuing up to and including end, with each successive date being step days after the previous one.

For example, if you have:

• Start date: "2023-04-01"
• End date: "2023-04-04"
• Step: 1

The generator would yield: "2023-04-01", "2023-04-02", "2023-04-03", "2023-04-04"

If the step were 2 instead, it would yield: "2023-04-01", "2023-04-03"

The key requirements are:

1. Use a generator function (function* syntax in TypeScript/JavaScript)
2. All yielded dates must be strings in "YYYY-MM-DD" format
3. Include both the start and end dates in the range (if the end date is reachable with the given step)
4. The generator stops when the next date would exceed the end date

Intuition

The core idea is to treat dates as objects that we can increment by a certain number of days, then convert them back to the required string format for output.

Since we need to produce values one at a time (lazily) rather than all at once, a generator function is the natural choice. Generators allow us to pause execution after each yield and resume when the next value is requested.

The approach follows these logical steps:

1. Convert strings to Date objects: JavaScript's Date objects have built-in methods for date arithmetic, making it easy to add days. We convert both start and end strings to Date objects for easier manipulation.

2. Use a loop with a moving pointer: We maintain a currentDate variable that starts at the beginning and moves forward by step days each iteration. This is similar to iterating through an array with i += step instead of i++.

3. Yield formatted dates: At each valid position (where currentDate <= endDate), we convert the current Date object back to the "YYYY-MM-DD" format and yield it. The toISOString() method gives us a format like "2023-04-01T00:00:00.000Z", and we can slice the first 10 characters to get just the date portion.

4. Increment and check bounds: After yielding, we add step days to the current date using setDate(currentDate.getDate() + step). The loop continues as long as we haven't exceeded the end date.

This approach leverages JavaScript's date handling capabilities while using a generator to provide memory-efficient, on-demand date generation.

Solution Approach

The implementation uses a generator function to create an iterator that yields dates on demand. Let's walk through the solution step by step:

1. Function Signature

function* dateRangeGenerator(start: string, end: string, step: number): Generator<string>

The function* syntax defines a generator function that returns a Generator<string> object.

2. Date Initialization

const startDate = new Date(start);
const endDate = new Date(end);
let currentDate = startDate;

• Convert the input strings to Date objects for easier manipulation
• Initialize currentDate as the starting point for iteration

3. Main Loop Logic

while (currentDate <= endDate) {
    yield currentDate.toISOString().slice(0, 10);
    currentDate.setDate(currentDate.getDate() + step);
}

The loop continues as long as currentDate hasn't exceeded endDate. In each iteration:

• Yield the formatted date: currentDate.toISOString() produces a string like "2023-04-01T00:00:00.000Z". We use .slice(0, 10) to extract just the date portion "2023-04-01".

• Increment the date: currentDate.getDate() returns the day of the month (1-31). Adding step to it and passing to setDate() moves the date forward. JavaScript's Date object automatically handles month and year rollovers (e.g., adding 5 days to March 29 correctly gives April 3).

Key Implementation Details:

• The comparison currentDate <= endDate works because JavaScript Date objects can be compared directly using relational operators
• The generator automatically stops when the loop ends (no more values to yield)
• Each call to .next() on the generator resumes execution from where it last yielded
• The generator maintains its state between calls, remembering the current position in the date range

Time Complexity: O(n) where n is the number of dates yielded ((end - start) / step + 1)
Space Complexity: O(1) as the generator only maintains a single current date in memory at any time

Example Walkthrough

Let's walk through a concrete example with:

• start = "2023-04-10"
• end = "2023-04-15"
• step = 2

Step 1: Initialize the dates

startDate = new Date("2023-04-10")  // April 10, 2023
endDate = new Date("2023-04-15")    // April 15, 2023
currentDate = startDate              // April 10, 2023

Step 2: First iteration

• Check: currentDate (Apr 10) <= endDate (Apr 15)? ✓ Yes
• Yield: "2023-04-10" (from toISOString().slice(0, 10))
• Update: currentDate.setDate(10 + 2) → currentDate becomes April 12, 2023

Step 3: Second iteration

• Check: currentDate (Apr 12) <= endDate (Apr 15)? ✓ Yes
• Yield: "2023-04-12"
• Update: currentDate.setDate(12 + 2) → currentDate becomes April 14, 2023

Step 4: Third iteration

• Check: currentDate (Apr 14) <= endDate (Apr 15)? ✓ Yes
• Yield: "2023-04-14"
• Update: currentDate.setDate(14 + 2) → currentDate becomes April 16, 2023

Step 5: Fourth iteration

• Check: currentDate (Apr 16) <= endDate (Apr 15)? ✗ No
• Loop ends, generator completes

Final output sequence: "2023-04-10", "2023-04-12", "2023-04-14"

Notice how April 15 (the end date) is not included because our step of 2 days from April 14 would take us to April 16, which exceeds the end date. The generator correctly stops before yielding any dates beyond the specified range.

Solution Implementation

Time and Space Complexity

Time Complexity: O(n) where n is the number of dates in the range, calculated as ⌈(endDate - startDate) / step⌉ + 1.

Each iteration of the generator performs constant time operations:

• yield statement with toISOString().slice(): O(1)
• Date comparison currentDate <= endDate: O(1)
• Date increment with setDate() and getDate(): O(1)

Since the generator yields exactly n values (one for each date in the range with the given step), the overall time complexity is O(n).

Space Complexity: O(1)

The generator uses constant extra space regardless of the date range size:

• Three Date objects (startDate, endDate, currentDate): O(1)
• No additional data structures that grow with input size
• The generator yields values one at a time rather than storing all dates in memory

The space complexity remains constant because generators are lazy and don't pre-compute or store all values. Each value is computed on-demand when next() is called.

Common Pitfalls

1. Date Object Mutation in JavaScript/TypeScript

In the JavaScript/TypeScript implementation, a critical pitfall is that Date objects are mutable, and the line currentDate.setDate(currentDate.getDate() + step) modifies the original object. If someone tries to store or reference the yielded dates, they might encounter unexpected behavior:

// Problematic approach - storing references
function* dateRangeGenerator(start, end, step) {
    const startDate = new Date(start);
    const endDate = new Date(end);
    let currentDate = startDate;  // This creates a reference, not a copy!
  
    while (currentDate <= endDate) {
        yield currentDate;  // Yielding the object directly
        currentDate.setDate(currentDate.getDate() + step);
    }
}

// This would cause all collected dates to show the same final value
const dates = [...dateRangeGenerator('2023-04-01', '2023-04-04', 1)];
// All elements in dates array would reference the same mutated object

Solution: Always create a new Date object or yield the string representation immediately:

function* dateRangeGenerator(start, end, step) {
    const startDate = new Date(start);
    const endDate = new Date(end);
    let currentDate = new Date(startDate);  // Create a copy
  
    while (currentDate <= endDate) {
        yield currentDate.toISOString().slice(0, 10);  // Yield string immediately
        currentDate.setDate(currentDate.getDate() + step);
    }
}

2. Timezone and DST Issues

Both implementations can face issues with timezone handling and Daylight Saving Time (DST) transitions:

# Problem: Using local timezone can cause unexpected date skips during DST
from datetime import datetime, timedelta

start_date = datetime.strptime('2023-03-11', '%Y-%m-%d')  # DST transition date in US
# Adding days might not work as expected around DST boundaries

Solution: Use date-only operations or UTC to avoid timezone complications:

from datetime import date, timedelta

def date_range_generator(start, end, step):
    # Use date objects instead of datetime to avoid time components
    start_date = date.fromisoformat(start)
    end_date = date.fromisoformat(end)
    current_date = start_date
  
    while current_date <= end_date:
        yield current_date.isoformat()  # Returns YYYY-MM-DD format
        current_date += timedelta(days=step)

3. Large Step Values Causing Month/Year Overflow

When using large step values, the JavaScript setDate() method handles overflow automatically, but this behavior might not be immediately obvious:

// If currentDate is January 30 and step is 5
currentDate.setDate(30 + 5);  // Becomes February 4 (or 5 depending on leap year)

While this is generally correct behavior, developers might not expect automatic month/year rollovers.

Solution: Document this behavior clearly or use a more explicit date arithmetic library:

// Using explicit date arithmetic for clarity
function* dateRangeGenerator(start, end, step) {
    let currentDate = new Date(start);
    const endDate = new Date(end);
  
    while (currentDate <= endDate) {
        yield currentDate.toISOString().slice(0, 10);
        // More explicit: create new date with added milliseconds
        currentDate = new Date(currentDate.getTime() + step * 24 * 60 * 60 * 1000);
    }
}

4. Invalid Date String Format

Both implementations assume the input strings are valid and in the correct format. Invalid inputs will cause runtime errors:

# This will raise ValueError
date_range_generator('2023/04/01', '2023-04-04', 1)  # Wrong date format
date_range_generator('2023-13-01', '2023-04-04', 1)  # Invalid month

Solution: Add input validation:

def date_range_generator(start, end, step):
    from datetime import datetime, timedelta
  
    try:
        start_date = datetime.strptime(start, '%Y-%m-%d')
        end_date = datetime.strptime(end, '%Y-%m-%d')
    except ValueError:
        raise ValueError(f"Invalid date format. Expected YYYY-MM-DD")
  
    if start_date > end_date:
        raise ValueError("Start date must be before or equal to end date")
  
    if step <= 0:
        raise ValueError("Step must be a positive integer")
  
    current_date = start_date
    while current_date <= end_date:
        yield current_date.strftime('%Y-%m-%d')
        current_date += timedelta(days=step)