Xelix Software Engineer
Interview Questions & Guide 2026

import re
from decimal import Decimal, localcontext


def minor_units(text, scale):
    if type(scale) is not int or not 0 <= scale <= 4:
        raise ValueError("scale must be an integer from 0 to 4")
    if (not isinstance(text, str) or len(text) > 64
            or re.fullmatch(r"-?(?:0|[1-9][0-9]*)(?:\.[0-9]+)?", text) is None):
        raise ValueError("invalid amount syntax")
    with localcontext() as context:
        context.prec = max(28, len(text) + scale + 2)
        scaled = Decimal(text) * (Decimal(10) ** scale)
        if scaled != scaled.to_integral_value():
            raise ValueError("amount exceeds currency precision")
        return int(scaled)


assert minor_units("12.340", 2) == 1234
assert minor_units("-0.25", 2) == -25
assert minor_units("500", 0) == 500

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CREATE TABLE invoices (
  tenant_id TEXT, invoice_id TEXT, currency TEXT, gross_minor INTEGER,
  PRIMARY KEY (tenant_id, invoice_id)
);
CREATE TABLE payments (
  tenant_id TEXT, invoice_id TEXT, paid_minor INTEGER
);
INSERT INTO invoices VALUES
  ('a','i1','GBP',10000), ('a','i2','USD',5000), ('b','i1','GBP',7000);
INSERT INTO payments VALUES
  ('a','i1',3000), ('a','i1',2000), ('b','i1',8000);

WITH paid AS (
  SELECT tenant_id, invoice_id, SUM(paid_minor) AS total_paid
  FROM payments
  GROUP BY tenant_id, invoice_id
)
SELECT i.tenant_id, i.invoice_id, i.currency,
       i.gross_minor - COALESCE(p.total_paid, 0) AS outstanding_minor
FROM invoices AS i
LEFT JOIN paid AS p
  ON p.tenant_id = i.tenant_id AND p.invoice_id = i.invoice_id
ORDER BY i.tenant_id, i.invoice_id;

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CREATE TABLE invoices (
  tenant_id TEXT, invoice_id TEXT, gross_minor INTEGER,
  PRIMARY KEY (tenant_id, invoice_id)
);
CREATE TABLE payments (
  tenant_id TEXT, invoice_id TEXT, paid_minor INTEGER
);
CREATE TABLE alert_events (
  tenant_id TEXT, invoice_id TEXT, alert_id TEXT
);
INSERT INTO invoices VALUES ('a','i1',10000), ('b','i1',9000);
INSERT INTO payments VALUES ('a','i1',5000), ('a','i1',5000), ('b','i1',1000);
INSERT INTO alert_events VALUES ('a','i1','x'), ('a','i1','y');

WITH paid AS (
  SELECT tenant_id, invoice_id, SUM(paid_minor) AS total_paid
  FROM payments GROUP BY tenant_id, invoice_id
), alerts AS (
  SELECT tenant_id, invoice_id, COUNT(*) AS alert_count
  FROM alert_events GROUP BY tenant_id, invoice_id
)
SELECT i.tenant_id, i.invoice_id,
       COALESCE(p.total_paid, 0) AS total_paid,
       COALESCE(a.alert_count, 0) AS alert_count
FROM invoices AS i
LEFT JOIN paid AS p
  ON p.tenant_id = i.tenant_id AND p.invoice_id = i.invoice_id
LEFT JOIN alerts AS a
  ON a.tenant_id = i.tenant_id AND a.invoice_id = i.invoice_id
ORDER BY i.tenant_id, i.invoice_id;

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